The Growth Detective
Most people use Duolingo to learn Spanish. Rosie Hoggmascall uses it to study paywalls. Where an ordinary user sees a streak reminder, she sees a retention mechanic. Where others see a free trial screen, she spots a monetisation decision - and a question worth investigating.
That instinct - to peel back the interface and ask why - is what defines Rosie's career. She has spent the better part of a decade inside some of London's most interesting consumer startups, doing the unglamorous work of turning curious users into paying ones. First at what3words, then at femtech startup Ferly, then at Peanut - the social app for mothers. At each stop, she sharpened the same set of tools: onboarding, activation, paywall design, A/B testing, retention loops.
Now she is Chief Product and Growth Officer at Fyxer AI, an email productivity tool that has grown from $1M to $30M ARR in a single year. That is not a rounding error. And the number that tells you more about the product than the revenue figure: 90% retention at month three. In an era when most apps leak users like a broken pipe, Fyxer keeps them.
But Rosie does not just do this work. She writes about it - obsessively, rigorously, in public. Her newsletter, Growth Dives, goes out weekly to 2,000+ subscribers. The average email newsletter achieves roughly 20% open rates. Hers hovers around 64%. That is not a typo. That is what happens when the writing is actually useful.
I write to learn and get deeper on certain topics. How? By reverse engineering the products of leading tech companies.- Rosie Hoggmascall, on her writing philosophy
A Cambridge Grad Who Chose Startups Over Suits
Rosie came out of Cambridge and walked into Kekst CNC, a strategic communications firm where she managed campaigns for FTSE100 companies. It was good work. Careful work. The kind of work that looks impressive at dinner parties.
She lasted two years. By late 2019, she had joined what3words - the location technology startup that assigns three-word addresses to every square meter on earth. There, for the first time, she encountered growth in its raw form: SQL queries, DataStudio dashboards, A/B tests, the perpetual question of why users were not doing what you hoped they would do.
What followed was a pattern she would repeat: join a startup early, build something from scratch, leave with a new scar and a new skill. At Ferly, a femtech app focused on sexual wellness, she was the first growth hire. She monetised the product. She ran multi-channel campaigns. She learned, as she later put it, what it actually means to start a growth function from zero.
At Peanut - a Series A app connecting mothers - she expanded into activation and retention, the two problems that make monetisation possible. And somewhere in the middle of all this, in 2022, she started writing on Medium. Not to build an audience. Not to launch a personal brand. To think.
Six Years, Six Stops
From $1M to $30M ARR - and Why 'No' Is the Secret
Fyxer AI is an email productivity tool that does what most software promises and few deliver: it genuinely saves you time. Under Rosie's watch as CPGO, the product hit 90% month-three retention - a figure that makes most SaaS founders quietly envious.
How? Rosie wrote a detailed breakdown. The answer is not what most people expect. It is not AI features, not clever onboarding gamification, not a viral referral loop. It is something simpler and harder: saying no. Hundreds of times per week. The product team at Fyxer made an active, repeated choice to decline new features in favour of making the core job - handling email - almost unreasonably good.
"New features rarely drive retention improvements," she wrote. "Winning retention demands obsessing over the core job rather than pursuing flashy additions." It sounds obvious. Almost no one actually does it.
The result: users who stick around. Users who bring colleagues. Users who become the kind of advocates that no performance marketing budget can buy. Fyxer went from $1M ARR to $30M ARR in 2025. That is what real retention looks like at scale.
New features rarely drive retention improvements. Winning retention demands obsessing over the core job rather than pursuing flashy additions.- Rosie Hoggmascall, on Fyxer's 90% month-3 retention
Growth Dives Newsletter vs. Industry Benchmarks
Growth Dives: Where Product Teardowns Meet High Open Rates
In September 2024, Rosie launched Growth Dives - a weekly Substack newsletter built around annotated teardowns of leading tech products. One year in, she had 2,000+ subscribers, 9,982 emails sent, and a 64% average open rate. She also secured her first sponsor, Attio, just one week before the anniversary. Timing has a sense of drama.
The format is deliberate. Each issue takes one product - Duolingo, Spotify, Loom, Canva, Claude, ChatGPT, Perplexity - and disassembles it. Paywalls, onboarding screens, pricing pages, cancellation flows. The kind of UX decisions that ship quietly and affect millions. Rosie labels them, annotates them, and explains what they mean for anyone building a subscription product.
Before Growth Dives, there was Medium. She wrote 87 articles over two and a half years. The pieces accumulated 720,000+ views. She earned $11,000. Then, in 2025, she left - because Medium removed the ability to export email subscribers. For a writer who understands retention mechanics better than almost anyone, ceding ownership of her audience was not an option.
"Virality is the only reliable path to earn on Medium," she wrote in her farewell piece. "And that is impossible to produce consistently." So she moved to Substack, took her audience with her, and started controlling the relationship directly.
Articles Worth Reading
How Duolingo pushes users from freemium to premium
Time to magic moment: Claude, ChatGPT & Perplexity
Strava's bumpy road to monetisation
How Figma's pricing is anti PLG
Spotify Wrapped: a UX review
The story behind Fyxer's 90% month-3 retention
7 examples of product-led growth in mobile apps
3 ways to reduce time to magic moment - lessons from Loom, Canva & Otter.ai
How Google Maps mines for assumptions
After 3 years and $11,000, I'm leaving Medium
I cancelled my Calm membership. Here's all the emails...
Reducing time to aha! moment in B2B SaaS
The Quotables
"Not doing enough mobile user research - sometimes people just listen to app reviews and support tickets. And if that is your user research, it's not enough."
Business of Apps Podcast, Episode #156"Getting something coherent on paper and freeing up brain space for deeper thinking."
On why she writes"Virality is the only reliable path to earn on Medium - and that is impossible to produce consistently."
Medium farewell article, 2025"Saying 'no' hundreds of times a week."
On the discipline behind Fyxer's retentionThe Details That Define Her
Early in her career, a manager took credit for Rosie's work - and when she confronted them, they threatened her job. She described feeling "insecure, psychologically unsafe, and like an idiot." That experience did not break her. It became the foundation of an expertise she now shares openly: how to handle toxic management, how to have difficult conversations, how to manage up. The Delivering Value podcast dedicated a whole episode to the topic with her.
She spent three years on Medium. Wrote 87 articles. Watched 720,000+ people read her work. Earned $11,000. And then - after a platform change stripped authors of email subscriber exports - she packed up and left. Not with a complaint. With a detailed, annotated post-mortem that became one of her most-read pieces. The irony of a growth expert using her own departure as a content strategy is not lost on anyone.
Her Twitter handle is @Rhoggmascall. She has consulted for companies backed by Index Ventures, EQT Capital, Alumni Ventures, and Intel Capital - a portfolio that covers some of the better-known consumer apps of the last decade. She mentors on GrowthMentor. She speaks at conferences. She disassembles products for fun and calls it research.
Cambridge-educated. London-based. French-speaking. The kind of person who treats a poorly designed paywall the way a detective treats a crime scene.
Achievements
- Led Fyxer AI through $1M to $30M ARR growth in 2025 as Chief Product & Growth Officer
- Achieved 90% month-3 retention at Fyxer AI - a top-decile result for consumer SaaS
- Sustained 10% weekly growth at Fyxer since product launch
- Named App Leader 2023 by Business of Apps
- Named Top Testing Influencer 2023
- 64% average open rate on Growth Dives newsletter (vs. ~20% industry average)
- 2,000+ subscribers within year one of Growth Dives
- 720,800+ total views across 87 Medium articles
- $11,000+ earned through Medium Partner Program over 2.5 years
- Speaker at App Promotion Summit London 2023
- First growth hire at Ferly - built entire growth function from scratch
- Fractional growth lead for Blinkist, Miro, Thriva, and SEMrush
Core Expertise Areas
The Portfolio
From pre-seed femtech to enterprise SaaS, Rosie has worked with products across the full funding spectrum.
Investor-backed experience includes: Index Ventures, EQT Capital, Alumni Ventures, Intel Capital