If you’re looking for a historical metaphor to understand the influx of users from Twitter to Mastodon, “colonization,” “refugees,” “gentrification,” and even “white flight” aren’t really appropriate. I’ve used “exile” but it’s imperfect too.
One that might be worth trying is “enclosure.” It’s a LITTLE like what happened when English landlords enclosed the commons and huge numbers of people abandoned farms for cities: https://en.m.wikipedia.org/wiki/Enclosure