Foundry isn't the only VTT. And I don't think it is the most popular, but we don't actually have any way to know unless everyone releases their user numbers.
Maybe I am not being clear. I am not saying that there is no place to play PF2E online with decent support. I am saying that in general when you try and get folks to play other games, in addition to all the usual problems of convincing them to do something other than 5E, you now have the added difficulty of "the VTT implementation isn't as good as D&D!" And it won't be, because more people are using and modding VTTs for 5E than any other system, by a wide margin.