I know the article I linked to ends on a fuzzy note, I'd have ended it quite down. Basically, they have no idea how to revenue their service. And they're really only reaching <20million people anyways (in theory, word on the street is that only a fraction stay as active users).
If you ignore the typical VC funded fantasy growth projections, when their internal revenue figures for q3 of 2009 were leaked, they weren't exactly rosy. http://techcrunch.com/2009/07/15/twitters-financial-forecast...
I think even more dysmal is their user growth curve. As of the beginning of this year it looked like this http://techcrunch.com/2010/01/19/state-twittersphere-hubspot...
Now, about revenue? it seems more related to social analytics, since using the API (with its limitations) it's difficult to do big scale analysis.