• Self-serve advertising allows marketers to decide precisely who they want to appeal to, and buy ads to put in front of users who fit the profile. This has already proved appealing for some big brands: right now, I can see ads from 3 Mobile and Peugeot on my Facebook homepage, for example.
• It's not just big advertisers, either. Almost anyone can walk up and buy space on the site if they have the cash - and that includes other Facebook users, who try to direct you to their profiles, fan pages or elsewhere in order to get you to follow their cause or buy their product.
• There are also the gifts and other virtual property that users can buy and give to each other. This still seems like a crazy idea to some people, but it can prove a highly profitable business in the right circumstances - virtual goods prop up all kinds of online games, and nobody ever thought ringtones could be so profitable.
