It must at least partly be due to the fact that Twitter has been guarding the simplicity of its product. Facebook is a communications platform at its core, but it also allows third-party developers to use it just as an identity database
on its own property. You can run games on Facebook.com, you can't on Twitter.com.
The simplicity of Twitter has also made them discourage third-party development off their property. If all you've got is a SMS broadcasting service and you have an open API, it's not so hard for others to clone all your features and more. It's not as easy to clone Facebook because it has a lot more functionality. So that has limited Twitter's off-site monetisation options.
Twitter is similar to Facebook in the sense that people spend a lot of time on it, but for some reason (good for users, bad for short term profitability), Twitter has not added any Facebook-style display advertising on its website.
Now, this guarded simplicity exists in another internet giant - Google search. You cannot really make any third-party products on top of it and you're unlikely to see flashing banner ads there any time soon. But, Google has intent, which means Adsense works great. While Twitter doesn't have as clear an intent signal, an Adsense-like product on it won't be a big surprise.
The simplicity of Twitter as a social network gives it a lot of pace. Stories, trends, memes move fast on Twitter. So, it's hard for advertisers to capitalise on any particular theme for long or at the right time. But, I expect they will churn out more brand management and marketing tools for big companies. That's what I guess Twitter's major source of revenue will be (big deals), rather than self-serve ad platforms for small to medium companies like Facebook and Google.