James Surowiecki in the New Yorker:
From a business perspective, the willingness of consumers to take risks means that new technologies can see profit faster here than they can elsewhere. That encourages inventors to invent, and investors to pour money into startups. (It’s no coincidence that the modern venture-capital industry got its start here.) And the speed with which successful products are taken up also allows companies to benefit from economies of scale sooner, bringing prices down and making it easier to reach even more customers.
Essentially, perhaps it wouldn't matter if Facebook or Zynga or Dropbox were UK companies (assuming they were reliant on UK consumers only). In other words, the US is how to build your audience for something new.
Incidentally, yesterday Ian Livingstone asked how we could have the next Zynga be British. I'd say it's already been and sold itself to EA for a large amount of money: Playfish. And related to the above, they focused on the US from day one.