I don't see how it foretells any plans to exit. Perhaps Ev just doesn't have the desire to be CEO of a 300+ person company. It's quite possible Costolo was hired as COO as a way of transitioning him into the CEO spot if he did well. That would certainly seem like a pretty smart strategy.
I believe your information is out of date. In addition to the two major data customers in Google and Microsoft (rumoured to be in the region of $25m/year) it also has advertisers who pay for promoted tweets and promoted hashtags. As for the "at the drop of a hat", of course they could, but why would they?