It’s all in the ads, as marketing investments are moving from traditional channels to search engines and Web magazines etc and then over to social networking, and because there’s not much else to make revenue from right now, with a distinct exception for micro payments in social games.
Note the increase for Facebook. Note also the lack of Twitter (specifically) in the pie chart. Will it show up next year?
Social Networking Ad Spending Over the Years – Career Explorer
The ad dollars go where the users go and where it’s possible to target the ads the best. The latter is what social networking brings to the table, as users are naively honest about the information they disclose about themselves.
This is probably also what makes it so hard for Twitter to decide on an ad strategy, as they don’t have any solid information about users. They could of course start with banner ads in the Twitter app and on the site, like everybody else.