Do you know why Facebook paid a billion dollars for Instagram? Instagram wasn't worth that much. It wasn't generating a billion dollars in revenue. It probably still doesn't.
Facebook bought Instagram because Instagram was a growing app that was popular with a demographic Facebook wanted to control. They spent a billion dollars to eliminate a growing threat.
Mastodon and, to a lesser extent, Lemmy, represent a growing threat. Not a very big one right now, but it could become a bigger one. It could become another billion dollar problem for the goliaths on the Internet, in a few years. They need to have total control, if a social media app starts to fragment it just collapses instead as users decide to go wherever the other users are.
Facebook's 1000:1 user ratio would make Lemmy irrelevant and stave off that billion dollar problem for Facebook down the road. An incredibly cheap way to kill a tiny but growing competitor.