Oops, I think I misunderstood what was being said. Sure, if PadMapper became the main place renters go, then it could pose a risk for Craigslist. I don't see that as likely, though - it's well structured for the technically literate, but my impression is that it's somewhat confusing for many of those who aren't. To add to that, it's extremely hard to build a brand strong enough to reach the landlords of the world, who are on average older and less likely to be early adopters of hip web technologies. Anecdotally, most PadMapper users tell landlords that they found the place on Craigslist, because it would confuse them to say "PadMapper". So, in my opinion, it's unlikely to be a real threat. Hope that makes sense.
Perhaps, the dynamics of a system like that are impossible to fathom beyond abstractions, especially if you've never observed a similar situation (I haven't). If it asymptotically reaches a relatively small subset of all renters, and it never becomes the main place renters go, then I don't think there will be a tipping point.
