If you have a look at the Y Combinator startups (at http://yclist.com), quite a large proportion of the 2008 and earlier startups appear to be dead. Although it only lists "Dead" for a some of them, if you click on the links for the other sites (apart from the ones that say "Exited") you'll see a lot of their websites don't exist any more. It looks like about 80-90% are dead.