When I started on twitter, it had no hashtags, replies were public (because they weren't initially part of the protocol), no t.co, no embedded photo sharing, no geotagging. All of these features were added, and made popular, in third party clients before being added to Twitter. It was like a giant lab where the features could be tried by smaller groups who shared certain clients or norms and the popular ones wrapped into the "official" API eventually.
