Returning as in people who left a long time ago but come back once they see their "failed social network" becomes big enough to be relevant in their lives again? I don't imagine that it'll be a very big percentage, but who knows.

We could build a bot to check it haha, every time there's a post, read if the user's last post was more than 3 months ago or if they joined more than 3 months ago. If so, add the user to returning user. If the account is less than 3 months old, new account? I don't know. Metric tags are subjective!