Not to be too flip, but because someone's prepared to pay (proportionally) that much for it.

Why are they prepared to pay that? Maybe they know something you don't, like Twitter's business plans, their current revenue, project revenue growth, those sorts of things.

It's easy for people to say that Twitter has little-to-no value because they can't think of ways to make money out of it - but that only means that they shouldn't be in charge of Twitter, not that someone else couldn't turn a profit on it.