Hi. For me any between facebook and twitter is best in promoting your site. In my opinion, the success of using any of social networks depends on what you are promoting and how you promote it. Thinking of the right strategy to market your business using social networks is what matters.
Social networking-new means of communicating and sharing information between two or more individuals on an online community. These are most Useful sites for Social Networking face book Twitter Stumble upon.com Squidoo Linked-in My-space Style.Fm
In the present scenario people are very much familiar with facebook,twitter because they very easy to handle and I am very much sure whenever a person seats in front of system he or she will definitely visit facebook and twitter so these two are the best social networking site. Digg is also helpful cause it helps to bring link for the site.
Nowadays social networking sites are increase every year. But, my choice twitter, stumpleupon and digg are best.