If you check the Top Users section of the site you'll get a numbered result of all the people registered on the site. As of the time that I'm writing this here's the amount of registered users per Whiskey Media site except Tested who no longer feature a list of users since they re-designed their site (Or maybe I just can't find it).
Giant Bomb (launched July 21, 2008) = 151 147 users
Comic Vine (launched December 6, 2006) = 124 775 users
Anime Vice (launched April, 2008) = 31 571 users
Screened (launched May 12, 2010) = 25 594 users
At first I was shocked. I thought: this few? But then I thought: wait, what do I know? Is 150 000 users a big number for an internet site? I have no idea really, though Screened's 25k seems very low. According to Whiskey Media there are 4 Million Monthly views across all of these sites. Obviously many people don't register but as registered numbers go can an educated person give me an idea of what context to place these numbers in?
UPDATE: I like the little spin-off topic that spawned from this. I am user number 23581, joined on Nov 18, 2008. That's 3 months 28 days after the full site launched and 8 months 12 days after the blog launched. Shame on me, all though I'm not sure that the blog featured user registrations yet?
Here's a way to see your user number, info provided by Moderator MB, I'm posting this below so we DON'T ALL NEED TO STALK HIM.
Right click your avatar, open image in new tab or copy image url, and it's the second number after uploads/ in the url (eg uploads/2/23581/ would be 23581). Should be the same on any image you've uploaded to the site.