After Facebook, what will be the web's next billion dollar exit? Let's build a virtual stock market for startups.

Assuming Yahoo! gets some kind of a deal done with Facebook (can you say motivated buyer?), we will have had three billion-plus acquisitions on the web (Skype, YouTube, Facebook) in the last year. 

So what's next?   Who has the best shot of creating a billion dollars of value?

At first I'd say Craigslist, but I don't really see Craig ever selling.  Perhaps he wakes up one morning and decides it needs a safe home just in case any pissed off New York real estate scammer vows revenge, but, regulatory albatross aside, I think its much more likely he'll "give it to the people" and maybe go public or something.   

LinkedIn?  I'm not sure they engage users along enough dimensions...  it is too much of a business tool rather than the go to social network for adult professionals.

What about  Revver or Spotrunner?  YouTube got sold because they dominated the medium and Google thought they could figure out how to monetize it.  What if these two figure out the monetization first and attract publishers and advertisers....  might be a necessary add-on... but maybe not for a cool billion?

Feedburner is dominating and monetizing a medium, with little competition...  if and when RSS takes off, they could start making bank in a hurry.

You know what would be interesting... and something I would definately monitarily contribute to the production of?  A virtual stock market for startups.  I need something to do in the fantasy baseball offseason.  I'd love to be able to buy and sell "shares" based on the predictive expectations of the crowd.  That might be a very cool experiment.   

Any other ideas for the next billion dollar acquisition on the web?   I don't think it will be us.  Our number is only $750 million.  :)