i think there is no optimization for the social bookmark sites. * do not spam social boomark sites (you may be banned because of this) * check nofollow (if you want them to help your sep results)
I'd say that usability, compatibility, participation, interaction and viral factors are some of the most important fields of social media optimization. This depends on the individual kind of social media though. If you want to get more specific information, here's an example for one field of social media optimization: Social bookmarking website optimization