No.1
Facebook
Facebook is a social network service and website launched in February 2004. As of January 2011, Facebook has more than 600 million active users.
Click Here to visit Facebook

No.2
StumbleUpon
StumbleUpon is a form of web search engine that finds and recommends web content to its users. Its features allow users to discover and rate Web pages that are personalized to their tastes and interests.
Click Here to visit StumbleUpon

No.3
Twitter
Twitter is a website which offers a social networking and microblogging service, enabling its users to send and read messages called tweets. Tweets are text-based posts of up to 140 characters displayed on the user's profile page.
Click Here to visit Twitter