FWIW, the number of available Free Realms servers has not increased in the past few months. (At 10 when I checked last week.) When Free Realms first launched in April, the number of servers grew quickly, at a rate of about 2 per week. It's been at 10 since around the end of May.
Unless they're upgrading the capacity of each server, this indicates that the number of active players is remaining constant, so new people are entering at the same rate that people are quitting. But in a free-to-play game, the number of registered users = the number of active accounts, and that number increases monotonically.