My initial idea (to be edited, of course):
The second or third* post from a new counter results in a welcome PM saying that seven or eight* more posts and they could be in the counters society.
The entry requirement is at least 10* posts across all threads leads to an invite PM.
Members receive monthly* news PMs to promote the threads. And smaller competitions (see below).
Smaller competitions e.g. hundreds posts etc occur throughout, being promoted by newsletter.
Users who haven't counted in 1 month* receive a PM telling them that unless they post another number within a week, they're out of the society. NOTE, this is so the society is seen as exclusive/desirable, encouraging people to continue counting.
Spotify as leader oversees the society, with several deputies stewarding a thread each, monitoring post numbers every week and sending subsequent PMs. If a deputy is unavailable for a week, Spotify as leader covers.
*These variables are subject to change.
Please note, this is by no means decided, hence this thread, to discuss these ideas, and develop them.