Facebook is world’s most popular social networking website getting really high traffic per day. According to Alexa Facebook is no. 4 website in world in terms of traffic it gets.  Facebook puts advertising in its site that does not look like ads, they look like part of their site. Facebook is also planning to sell their user data to corporation so they can advertise on your information based and you will see more targeted ads in Facebook.
Site like Facebook that get this type of high traffic from all over the world can easily earn lot of money, they don’t need to find advertisers, they just need to find out quality advertiser from huge list so they can keep Facebook beautiful.