I don't understand why any investor would be worried about missing an opportunity like Pinterest at this stage. Yes, they have a lot of traction and a nice valuation, but if you're an investor in Pinterest you have effectively nothing to show for it right now. If they go public successfully or sell for a large chunk of money, then you can kick yourself for passing it up; as it stands, you made a logical decision with the info you had, and that <i>may</i> have turned out to be the wrong one, for the right reasons.
Why is Pinterest such a shining example when it doesn't have a fully developed market model yet and hasn't shown a single profitable quarter? What makes it more than just another overvalued company waiting for the next burst?