Social network sites (eg myspace.com, digg.com, youtube.com) can be effectively used for Search Engine Marketing (SEM). Unlike more traditional methods of optimising a site, these Web 2.0 sites can prove more of a challenge. The potential audience is huge, yet fickle. So make sure you have something interesting to say to them!
Read this article which elaborates on this subject and includes commentary from some eminent SEM's currently attending the San Jose SES (Search Engine Strategies) conference.