I think it would depend on the social network you have. Generic social sites like MySpace are difficult to monetize. However, if you had something with more a focus, I think it would be easier. If you social site was all about shoe shopping, for example, you could put ads for shoes up and probably do pretty well.
I have a website that is catered to a specific demographic right now. The ads are extremely relevant but do not pay that much. It's basically a "made for adsense" website. I am pulling about 25-30 dollars a day. Like I said it's a MFA with good blending almost forcing a click. I average 1k uniques a day, resulting in 30k uniques a month. My traffic is climbing and my website is aging. Should I try to morph into a social network?