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.
I think the biggest benefit for Twitter was that thanks to their API there were developers working on improving an innovating on top of the product even when Twitter was focused entirely on scaling.
Yes, Twitter was lucky in more than one way. I agree with what you said about early adopter. I'd label these people as "willing to test products", even if that means they will encounter some bugs or issues. However, not every business will be able to say that about their first set of customers.
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.