And, like Lemmy, lacks a large userbase comparable to that of Twitter or Facebook
Sites with small user bases will never grow as long as everyone is waiting for everyone else to migrate before doing so themselves.
The problem is that the Network Effect keeps people trapped in the original social media network silos.
No matter how much you want to leave, if the other options simply don’t have enough users to meaningfully engage with the topics you’re interested in, then it’s very hard (but not impossible) to switch over.
A lot of people on Twitter might be part of micro-communities that simply don’t have any presence on Lemmy or Mastodon yet, or they might be there to follow a specific person’s updates on a given project, which can’t exist on the other platforms unless that person decides to migrate themselves (which necessitates losing access to the majority of your fanbase at the start, and hoping they’ll all choose to move over solely because of you.)
I’m not saying it’s a bad idea to migrate, but it’s not as easy as a lot of people make it seem by implying people can just switch whenever they want to without major consequence. For a lot of people, switching would mean losing an entire community completely, or being wholly cut off from the people and topics they were previously following. And as much as they might not want to support a social media oligarch, if they switch, they have a lot to lose before it possibly gets better for them.