It depends. I have some friends who sell retail stuffs using facebook, others use pinterest, and there's also twitter. If your target audience is more on people who are active online especially on social networks, then you go with any of the above social network sites. But, if you target those who doesn't really have time to browse their social network pages at all time, like those business executives and the like, then social networks might not be the best ones to be in. I just learned this from my boss as well.
Adwords rocks because it targets all audience, and not limited to social network users. Anyone browsing using Google can be a probable client.