I don't do any real tracking except for login times, so here is the best data i can offer (for mastodon.social exclusively, other Mastodon and GNU social instances are obviously excluded but should count towards your question because of federation):
1687 users active in the last 14 days. 16,744 registered accounts on mastodon.social. 18,347 accounts in total known to this instance. 460,563 statuses in the database, 306,905 created by locally registered users. 173 different Mastodon/GNU social instances known to this instance (i.e. at least one account from such an instance in the database).
