Can anyone explain to me how the people who run Twitter make money?

[Swampy_Bogtrotter]

From what I can see, Twitter is entirely free to use, it isn't propped up by advertising, and there are no visible revenue generators on the site.
The site has an enormous worldwide user base, with incredibly high traffic, and all that costs money, so they must be incurring fairly large running costs.

So, how on earth do they do it?
They must be making money from somewhere, but for the life of me, I can't see where or how.
Anybody got any suggestions?