That's almost what he did—he was one of the founders of Paypal. Not exactly a "social" app, but he definitely used his fortunes from something more mundane to fund his more daring ventures.

IMO Paypal could be described as social payment

Fortunately it was founded in more civilized, less buzzword-y age... ;)

I know. But maybe it's time to do it again :).

I think social apps are harder than crazy ambitious ideas. It's easier to see how one would work on them, that's for sure. But precisely for that reason we see many many social apps. But few are trying to build spacex or similar, so it's easier to make progress along the axis of figuring out ways to create value.