That's kinda a big deal, I'm not posting to counter you, but do you have a source? I'd love to see it please and thank you.
Earlier this year In May of 2022, Foundry VTT's 2nd anniversary, there were 52,000 foundry servers (each is a licensee). The numbers will have increased since then... how many? I don't know.
Previous percentage breakdowns of their userbase last May 2022 was 5e 65% PF2 20% PF1/SF 5%, with the remainder of 10% or so split between a multitude of smaller RPG systems.
Reverse engineer those numbers and having paid attention to who is playing and what they say they were leaving behind? Overwhelmingly, it's Roll20.
It's not even close. FG2 and FG Unity have lost customers to Foundry VTT as well, sure, but it's
much lower in number (the tech advantage difference between Foundry VTT and Foundry Unity is much more narrow than it is with Roll20 - and there is no importer to dl/migrate a campaign they had paid for from FG to Foundry VTT. With Roll20, you can migrate it all from Roll20 to Foundry VTT in 10 minutes or so.) More to the point, FGU is much more sophisticated software than Roll20 is, so the tech advantage Foundry VTT has over it is much smaller. Still, FGU does require a download and install (Foundry VTT is accessed instead via Chrome, like Roll 20 is.) It also requires purchases of game software and modules -- neither of which Foundry VTT requires. Foundry is therefore a superior substitute, which is also cheaper than both. That has the consequences in the marketplace you would expect it to have.
You can look at the numbers on Roll20 and they confirm this, in part. There was a dramatic exodus of PF1 and PF2 GMs (and Starfinder, too, though that was never large on Roll20) from Roll20 in 2020 and 2021. We don't have to guess what happened -- everyone in the business knows that Roll20 lost almost the entirety of their PF base (it's now only about ~3% of Roll20 users). And it's over 20% of Foundry's userbase.
We also know
why that happened. It was because Foundry VTT's PF2 code is robust and polished AF -- far more polished than its 5e code, to be blunt. Foundry's 5e code is created by the Foundry's main programmer Atropos (he now has a few programmer employees, but before late 2021, he didn't) and so Atropos had to code all of Foundry VTT PLUS 5e's game code as well. In contrast, the PF2 dev team for Foundry has ~8 hardcore members and over 2 dozen more part-time contributors and data entry volunteers. Even Atropos has admitted on Twitch/Youtube that the PF2 game code was more advanced and polished than 5e's (Why? Again, PF2's community simply have more programmers working on it). Add to that limitations which come with the SRD -- and no limitations which come with the OGL, and that ties it off with a ribbon.
But above all, the Foundry VTT module PDF2Foundry allows anybody who had a valid watermarked PDF from Paizo to instantly create and import that adventure, PFS Scenario, or Adv Path module into Foundry VTT for free (exception: it would not import monster art not contained in the adventure -- people got that from the Archives of Nethys for free, instead.) The bottom line was that if you are a Paizo subscriber and play via VTT, you will always choose Foundry VTT essentially by default as you get a watermarked PDF for the Paizo lines you subscribe to directly from Paizo as part of the bonus for subscribing directly. Add $50 for Foundry VTT as a one -time payment, and you had a slick VTT that supported your game without additional cost. That was a no brainer. So those subscribers left Roll20
en masse for Foundry VTT.
In the face of all of this and the runaway success PF2 was on Foundry VTT, Paizo officially licensed their APs and monster art to Foundry VTT. Those are now sold on Paizo.com. Since Foundry VTT became a Paizo licensee, the PDF2Foundry module is no longer updated and those modules released after Foundry became a Paizo licensee must be purchased specifically for Foundry VTT from Paizo. The previous stuff that was still free remains still free though.
Anyways, none of this is controversial. Foundry leapt past FG early in 2021 in terms of total customers and has been eating into Roll20's market share the most ever since. Sure, there are some new users to Foundry VTT who never had any prior experience on any VTT who came to the software in 2020 when it was released during the pandemic -- but that's a pretty small element of their userbase overall. Overwhelmingly, those customers used to be Roll20 customers and no longer are.
You can check all of the reviews and comparisons on Youtube, too. None of this is new or controversial -- it's been clear since 2020 that was the case. Foundry VTT is not only the most advanced VTT software currently on the market, it is also cheaper than even 1 year of Roll20 with Roll20's dynamic lighting enabled -- and Foundry VTT doesn't charge for game systems or adventures. On the 5e side, all of that licensed code not in the SRD flows to Foundry VTT through DDB without any need for an additional purchase (excepting those who pay directly for Mr. Primate's plugin on patreon. That's about 10k owners a month, as you yourself recently saw.) There is another importer, VTTA, that is essentially for free now which imports monsters from DDB as well. VTTA doesn't import adventures though. Home brewers go with VTTA; adventure module buyers go with Mr. Primate's patreon DDB importer.