If every spot was filled in every division with a registered user that would equal 4,096 teams.
On average there should be a minimum of 4 registered users per league on SL for there to be 1,000 users.
Flicking through the leagues, I don't think 1,000 users is a completely stupid figure and I would guess there's around 500.
Like Cove said, if you ignore multiple teams and exclude users that don't purchase credits, then the figure of active registered users can't be anywhere near 100.