I'm surprised that they have never offered a paid, pro version of Twitter. Even features as simple as no-ads and access to their complete timeline history would be enough to entice some people.
Revenue is a big no-no in silicon valley, it punctures the reality distortion bubble.
I wonder do their bills exceed $5 million a month. (I know, you pulled numbers from nowhere. I'm only making the point that they have an absolutely enormous userbase[0].)
[0]http://thenextweb.com/twitter/2011/09/08/twitter-100m-users-...
Business idea for twitter: paid access to the old, non ajaxified interface.