Despite rapidly growing concurrency rates, available evidence suggests that Second Life's core user population continues to stagnate at about half million-- almost exactly the number it was last June. 507,288 in September 2008, to be exact, compared to 507,844 in June 07.
507,288 is the number of Residents who logged into SL during September, minus the number of people who created a new SL account within that period. (On the company's official stats page, captured at left, Linden Lab lists the raw total of Residents logged in over various periods, without excluding new accounts.) But Massively's Tateru Nino has been diligently documenting the number of new accounts on her personal blog; 371,022 new accounts were created in September, she told me last Friday. Thus the subtractive math.
Last week, Linden Lab CEO Mark Kindgon told me that Second Life's "[A]ctive user base and active user hours have been growing nicely since the beginning of the Summer." So it's possible that these September numbers do not reflect the growth trend Kingdon mentions. I've requested precise recurring active monthly numbers from the Lindens, and will post them here, when/if I receive them.