how does sites like friendster,facebook, earn revenues?

[jepoi]

there are advertisements on the sites...those little banners, the sponsored links you see, those are all advetisements. the companies that want their advertisements displayed pay myspace/friendster/facebook to put their links/banners on the site, and revenue is made when someone clicks on a link. go to myspace's homepage now, u'll see advertisement at work all over the page.

its done with programs that register and keep track of where a user who was redirected to a certain page by clicking on a link originally found that link, and then the original site gets paid for how many times someone clicked on the link from their page