With so many amazing social networks and communities to choose from, there is one main golden rule I think people should follow, and that is to be a good neighbor. Think about your real community where you live. How would you like it if your neighbor spammed you with flyers everyday about ring tones, or viagra?
However; if you built up a relationship with your neighbors, and they started to "trust" you they will probably be more willing to check out the things you like (or are offering) and you will build up more friends this way. Social communities can be a powerful source of traffic when done right. The traffic can even be a higher converting traffic source than the normal SERPs.
It really does not matter what social network you fancy more - MySpace, Facebook or some other one, you just need to become the type of "neighbor" that everybody loves talking about, and not in a bad way. If you spend some time recommending good things, proving good information, linking out to people that you have met (and like), the love will be returned.
If you need me to spell it out for you - make sure that you are not just saving your own articles, or web pages to sites like Del.icio.us or Digg. Share other peoples "stuff" in the community that you like (they will notice) and other things you like related to what you are doing. Make friends, ask people to be your friend, give people reasons to come back to your profile and most importantly, do everything you can to get your face out in the community!