I think you should distinguish the different network sites when asking this question. You have the general sites ala Facebook but there are sites as xing.com, and [URL=“http://www.ravelry.com”]ravelry.com as well where the focus is way more specialized. You can’t compare such sites in the first place.
Obviously Facebook, but my best 2nd is Twitter. I have had great success with Twitter. Their search function is used a lot and searches related tweets about a topic. If you tweet frequently and people do a search for a keyword they might find you. Another thing is that if you tweet something with a particular keyword, search that keyword in google, it will show up for sometimes up to a minute in the Google News which is very powerful. It’s really short, but have a look and you will find related tweets falling through ;).
Every Social bookmarking sites have own specialty. But Facebook , twitter and digg are important and these are most traffic gainer then others social media sites