Well, couldn't that same argument have been made about Friendster, or MySpace, or Digg etc.?

I don't know what the odds are, but surely there's at least a 25% chance that twitter will fizzle out. So it's 25% chance of nothing, and 75% chance of, say, a 3x exit. I dunno, that seems crazy risky to me.

Someone commented below about the secondary market - that actually makes a lot of sense I think. I guess when you're investing in twitter there's enough liquidity in the secondary market that you can probably get out if you need to.