These are the best social networking according to my idea 1. Twitter 2. Facebook 3. Linked In 4. Digg 5. Delicious
You can use Yahoo answers for some good results i have experienced it and got good result from it also.
My top 5 Social networks are Facebook, Twitter, Plurk, LinkedIn, and Purebloggers. These are the best social network for me by means of networking and marketing aspects.