Kinda a silly example, but several subreddits that started out with the aim of making fun of some subject (i.e. r/prequelmemes and the Star Wars prequels, or r/the_donald and then Presidential candidate Donald Trump) were quickly turned into communities earnestly supporting the initial subject of parody.

I think Reddit is the broadest example, because it's evidence of both outcomes due to the diversity in moderation policy between subs.

Some can tolerate a steady influx of bad actors: some fall apart. There's probably a valid paper in there somewhere.

I don't think this is silly at all. And the fact that reddit's admins occasionally have to step in with a forceful hand over what the mods do only speaks louder to GP's point.