I understand what you mean, but network effects aren't unsurmountable. I'd argue that social networks are the pinnacle of services sustained by a network effect, but there's still fluidity (MySpace was a pretty terrible platform sustained by a strong network -- until it wasn't.)
> ...
> If PadMapper's approach really is so much better than why do they need to bootstrap themselves with Craigslist-affiliated data?
You already said it. Because of the network effect.