4. Twitter

With about 555 million users, Twitter is still far behind Facebook’s close to one billion user base. There are a lot of differences between the two social networks, the biggest according to a recent New York Times Bits blog, being trust. Twitter has never made its users' private information public while introducing new features and unlike Facebook, Twitter has not endlessly changed its privacy policy. Users trust Twitter more. This factor can be the strongest reason for users to switch from Facebook to a different social network.

5. So.cl

Microsoft's So.cl is not very famous as a social network at this point, but, with the recent announcement of increased social capabilities in Office, there is a potential for some kind of internal/external social integration with SharePoint. So.cl was initially introduced on college campuses, but is now open to anyone who wants to give it a try. It combines the elements of social networking, search and video sharing.