I think Twitter benefited from having no attractive competition during that time (mostly there were similar sites, with little innovation and few users).

Also I think you get a 6-9month grace from early adoptors while they can see you scaling up staff and getting funding. However, after a certain point they start putting up blog posts about the lack of progress and pointing out alternatives. I think this is partly because it takes a while to get really used to what your app. does and how it's useful to them.