It's a minor footnote, but I think another part of that meme that was really successful is that it had a theme that included things that almost all roleplayers (and their characters) experience, either in games ("characters turned into animals in their games/drs") or in canon (space travelers). This allowed people to go "hey, my character's done that!" and they have a much easier time hopping right on the tag.

It's also phenomenally easier to tag because unlike most memes, this one allows you to play your character without trying to set up the plausible scene, since the scene is already set in the body of the post. It's a support group, and that's pretty darn self-explanatory. We've all seen them in movies at the very least. All you have to do is set up what the support group is about and from then on, you tag absolutely easily, either straight from canon or right out of the game you're playing. It makes it a lot faster and more effortless to come up with reasons to tag other people.

Anyway, those are my extra two cents on it.