But the FB brand does not get your app Product Market Fit.
Those people who entered the door because of the ads $ spent, leave and then don't come back.
Never mind that your math doesn't even make sense. Even if only 1% of users retain after a year, 1% of 100m is 1m more than your 90% retention of 0 users.
Just the metrics and analytics that FB can gather on experiments is an insanely huge hill a regular startup can't climb. FB can gather and analise data at a scale, on so many markets and devices and demographics that none other plan tier at 3rd party integrations can hope to match. The SRE support you get, a 30 minute conversation with other great product managers, engineers, designers one meeting invite away, a history of what others have tried and what worked and what didn't, and so much more.
And if you get a VP to support you, even for a bit, we're basically talking about a torrent of money and resources being thrown at your project while you still drawn a 6 figure salary (plus benefits and bonus and stocks!). It's really apples to oranges here.
having experience at FB as a PM is not the only qualifier for starting a start-up. it takes a lot more than that to be successful