Also, it sounds like development was going on, improving architecture for scale and fixing bugs.
I've heard several people rave about rapportive so whatever they are doing is obviously working.
I feel like Twitter was very lucky that they did not see a decrease in use and adoption while they had many problems with scaling.
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.