As far as I know there is currently no way to migrate accounts, what I did was just scrolled through sub.rehab and signed up to all the ones I liked from there

You’re right that mastodon isn’t a huge community but fediverse social media is gaining traction and I think the sense is that by meta entering at this point can kill a lot of momentum going into mastodon/Lemmy etc. By it being a “mainstream” entry ino the fediverse they can consolidate the user base by virtue of it being where most people are, and having simple signups/interface/whatever then eventually shut out the rest of the fediverse, draining users to their ecosystem and leaving mastodon bare.