Of course they're doing it to mine user data - their primary business model is to run platforms to collect user data. They then sell user data both directly and by running the second largest targeted ad network.

Their public stance they made when renaming themselves meta is "we found out social networks have a lifecycle, and we want to get ahead of the curve and create/capture the platforms people are moving to"

There's plenty more to say about Facebook and big companies entering the fediverse but I kinda feel like anyone who is reading this understands the issue to a significant extent