"“Meta is hoping for at least tens of millions of users within the first few months of availability”. The Fediverse currently has between 1 and 2 million active monthly users. So Meta is expecting at least 10x of those numbers by the end of the year.
(If you think of it, of course they want those kinds of numbers. Both Facebook and Instagram have far more than a billion users each."
I'd offer a fourth possible reason: To kill Twitter.