And how many of those failures actually pay back investors instead of burning through all their cash first?

This is quite possibly the most responsible failure I've ever read about.

Odeo. Except that in hindsight, I think the investors may've wished that Ev kept their money and started Twitter out of the same company...

I know someone that tried to shut down his company (rfid luggage tags) and return capital to investors. He ended up being forced out and the company burned through all the cash and shut down. Sometimes doing the right thing isn't possible.

The 3 thirds figure is thrown around a lot: 1/3 of VC investments make a sizeable return, 1/3 break even, 1/3 crash and burn. This would appear to be the middle 1/3.