You were probably including the following in your meaning for "feature fit, timing, and growth", but just to echo / amplify your comment: two extremely difficul

You were probably including the following in your meaning for "feature fit, timing, and growth", but just to echo / amplify your comment: two extremely difficult things in building a community of Twitter's size (neither of which are technical / implementation in nature) are a) achieving network effects [1], and b) solving the Eternal September problem [2]. Both these hurdles are much more strategic than technical in nature. Andrew Chen's "The Cold Start Problem" goes into some fascinating detail about this [3]. 1. https://en.wikipedia.org/wiki/Network_effect https://en.wikipedia.org/wiki/Network_effect 2. https://en.wikipedia.org/wiki/Eternal_September https://en.wikipedia.org/wiki/Eternal_September 3. https://www.amazon.com/Cold-Start-Problem-Andrew-Chen/dp/0062969749 https://www.amazon.com/Cold-Start-Problem-Andrew-Chen/dp/006...