Purchasing a 50% random sample of all tweets in real time is reported to be $360,000 per year [1]. Apparently you can get the 100% feed for a negotiated price. If they can sell many licenses for those feeds, which I think that they can, they have the potential for real profit growth. I too was very skeptical of Twitter's profit potential until this service launched.
Do the math on this in your head --- many/most? of the Global 2k don't need a full Twitter feed; what's US Gypsum gonna do with it? --- and you're left with a total addressable market that doesn't come close to justifying their purported valuation.
I'm not being down on Twitter. Just, whatever the monetization plan is, this can't be it.
1. http://www.readwriteweb.com/archives/twitter_to_sell_50_of_a...