I am curious as to who pays the bills to keep the lights on around here? Is the site supported purely by advertisements? Does stackexchange itself own the site?
This was discussed on Meta StackOverflow: What is Stack Overflow's business model?
In short, they get money from Careers 2.0 and ads on SO, SF and a little SU.
Stack Exchange works primarily from VC funding. It raised 12 million USD in March' 11. This is what keeps the sites up.
The earnings from sources mentioned in Jeff's answer make up a portion of the earnings but alone, are not enough to keep the servers running and employees paid*.
Also, note that the company has expanded significantly since 2011 whereas ads have not increased in number. Stack Exchange is focusing mainly on expanding Careers and increasing earnings from it.
*I will add a source for this when I find it. I think it was mentioned in one of the podcasts.