If anyone still has any doubt that Fitbit's success is due to network effect, the opening paragraph of James' 2016 Q1 Earnings Conference Call should answer this:
Note how the unquestionable winner of activity tracker talked about defensibility as a direct result of network effects.
This following statement, made at 05'40" of the audio webcast, makes it even more clear:
These are all public statements that could easily be found. It sometimes surprised me that so many people in this industry still missed this part of the Fitbit success. As an entrepreneur it would be great sin not to at least understand how other people succeed, whether one chooses to go down the same route or not.