This is very interesting. I wish I could answer, but I honestly don't know. I'm well aware that this site makes money, although they don't charge an annual fee to the users. I have worked in so much retail, including the banking business, that I know whenever you see a "share with your friends on _____ *insert social networking media*" : it's a form of advertisement. They're trying to draw new clientele. I also knew for sure when a user here was given a position as GAG admin and she said it was now a "job", insinuating that she is getting paid for her services. Not sure where the money comes from though...
It primarily makes money via ads. High Xper-level users don't see ads, but they are a small minority - the majority see the ads, and the site gets a cut of anything anyone buys when they click on the ad, plus, likely, some money for raw ad views/clicks.
If you look at 20 questions a day here, that's 20 pages of ads that you see. Multiply that by several hundred thousand daily users, and that's a LOT of ad views, and potential clicks.
I still come across some ads on this site. Other than that, i really don't know how exactly this site is profitable. I can't see how this site would exist if it didn't make any sort of money. Good question by the way , i often wondered the same thing.