As I write this, it's just over 5.9 million registered accounts, so they'll hit 6 million in the next 48 hours or so.
Most of those aren't returning (as in at least once a month) users. At any given time there are between 20 and 30 thousand people on-line (including the first timers). The number of logins last 60 days is 1,75 million. In the past 90 days 2,1 million accounts were created, so let's assume 1,4 million in the past 60 days. That leaves you with 350.000 people in the past two months that were not first timers.
Those are relatively small potatoes compared with:
Cyworld: 19 million returning users in South-Korea alone
World of Warcraft: 8,5 million people with a monthly subscription (96 million US$ revenue per month)
Habbo Hotel: 70 million returning users worldwide, mainly in the age of 9-12
However, Linden Labs announced that they started turning a profit in January, so that is good for them.