Launched in February 2004, Facebook is a privately owned social networking site. facebook has 750 million active users as of July 2011.
Twitter is a social networking and microblogging service which allows you to send and receive text-based messages, in real time, known as tweets. Each message can contain up to 140 characters.
LinkedIn is a social network aimed at businesses only. Officially launched in May 2003 when it’s five founders invited around 350 of their important contacts to join. By the end of the first month, LinkedIn had 4,500 members in the network.
