You have to ask yourself: is it really worth developing an app that integrates with, or worse runs completely on Facebook's platform?"
If Facebook pays me: Sure.
They better be paying you incrementally for each user forever for all the data they collect from users that use your app or service... otherwise, you'd be a fool to base *anything* "on top of" the Facebook ecosystem.
I am constantly amazed that there are so many services that build upon Google, Apple or Facebook web authentication systems. It's just plain stupid for anyone to do that unless they are Google, Apple or Facebook as those services can eliminate your access to your customers ANY TIME they choose without you having any say in the matter.
And of the 3, Facebook is the worst, since by forcing users to have a Facebook account to use your service you are broadcasting how little you care about their ability to control any of their privacy given that tracking that you enable FB to perform against those users all over the net and FBs consistent history of altering their user terms to the detriment of their users.
If I see a service that REQUIRES a Facebook account, I will not use it whether it is free, paid or otherwise. And I am far from alone. Any developer that forces FB authentication in their apps or services is likely giving up at least 1/3rd of potential customer/users.