Fair question to ask! Although would agree with 'subset of Facebook' argument but therein also lies its beauty. If we look at Facebook today and filter marginal things away its used primarily to 1) Share updates and 2) Share pics. Twitter took the first point and made it better. Instagram took 2nd point and made it better. Both did well. So sometimes (in fact most of the times) its about taking only one feature and doing it extremely well and I will give credit to Twitter for this. As for the question related to a stable user base is concerned it feels people will find usage for both kind of platforms, general ones and very specific ones!