LinkedIn is 4 people you know. Facebook is 4 people you used 2 know. Twitter is 4 people you want 2 know.
Read hte full article at: http://www.irishrecruiter.com/linkedin-is-4-people-u-know-facebook-is-4-people-u-used-2-know-twitter-is-4-people-u-want-2-know/