The other day, someone posted a thread about instances that had just started suddenly having tens of thousands of members. That was the first I saw on the topic.

I went to one of the first ones on the list (k6qw or something like that) and it was a brand new instance with no communities, one post with one response, and 20,000 members.

So it's safe to assume they're either bots or red lectroids (have to see if they're all named John).