>> It's quite possible Costolo was hired as COO as a way of transitioning him into the CEO spot if he did well.

I'm quite sure this is what happened. Here's a detailed breakdown from Shervin Pishevar on how SGN did practically the same thing.

http://www.quora.com/Is-it-true-that-Shervin-Pishevar-is-out...

OTOH, Having 300+ people at some silly little micro blogging fad with very little revenue, doesn't seem like a pretty smart strategy.

So you downmodders think 300 employees is a reasonable number for a yet to be proved no business model startup? Wow.

Their quarterly revenue is, I believe, estimated somewhere in the region of $X million.

I suspect you got so many downvotes, though, because of the dismissive tone of your comment.

Their revenue is from 1, or at best 2 'customers', who could stop being 'customers' at the drop of a hat.

I believe your information is out of date.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?

Once wave takes off of course!