What everyone says doesn't completely answer the question. Yes it's about selling your data and attention to advertisers. But if it's about the "meta", than there is a twofold strategy about it: first exploiting the network effect (wikipedia link) while growing. And then locking in the market ("keep you in their ecosystem"), thereby locking out competition. It's ironic, but capitalists hate competition (in their own field) so much they would do everything to avoid it.

Their ideal endgame is what Amazon has achieved: becoming so big, they can start selling other capitalists access to their walled in market.

All these platforms could have been made compatible with each other (like federated instances). Without content walled in behind logins, we would be able to put together our own feed with content from all over the Internet and choose our own algorithms to sort it. But then no one could sell your attention or data to advertisers and small creative upstarts would be able compete with big entrenched content providers.