This is a bit of a silly response on your part. You're not answering the question of WHY people are on FB and not on the little sites like existed 20 years ago before FB. It's called the network effect. You have friends, your friends have friend, those friends have friends. Rather than there being 30 bajillion separate sites representing these friends connections, people go "hey, why not one site with everyone there".

Said little sites may run for a bit and die, and the massive monolith remains, at least until another monolith replaces them.

Well, indulge my silliness for a moment... what if the servers on the internet could talk to each other?

I suspect in just a few more decades, we shall reinvent the 90s and 2000s p2p networks from first principles.