My opinion is it’s just the data, Meta is all about collecting data. Being part of the fediverse means they potentially get access to a bunch of data through scraping and user interactions.

For example someone might not follow a gardening account on instagram/Facebook but they might join a gardening community. That’s valuable data.

They can also boast about the new technology. Maybe they hope it will revive the meta verse lol

I’ve got like 4 downloaded right now