If you're asking for just the data right now, this question is definitely too localized. So I'll assume you're actually asking for how to figure this out at any given point in time.
Unfortunately the answer to that is, you really can't. I am assuming by "user base" you mean "number of copies sold". The NPD tracks retail sales of games, but they can only estimate digital sales of games (source). Steam has specifically said that they don't want to release their actual numbers to the public (mostly because they prefer game companies pay them for access to the numbers). So the best you could do is trust the NPD estimates and add up the NPD numbers (this site lists the top 10 for each month) over the months that the game was available. This would give you an idea of how many copies have been sold.
But even then, you wouldn't be able to track the total number of users online. The Steam stats will only give you Steam users, so if you divide Steam users by all copies sold, you're going to get a much lower percentage than reality. You could improve on your estimate by finding other things that track number of users like XFire and adding them in.
So at the end of the day, you're going to come up with a number that has been through 5 levels of estimates, and probably isn't going to be useful for much of anything.