Twitter alternatives will all fail because they try to be an application for millions. They see Twitter and see the volume of users so build something for the numbers and hope that they are less bad. But Twitter and every other thing that has grown big has 1) started small 2) was not initially created to be big and 3) was attractive to use.
Twitter succeeded because (and I'm not au fait with the details of how exactly) they managed to onboard a lot of high profile users from a variety of different walks of life. Everybody else came to hear what they had to say. This must have been a huge amount of work, and not something that's talked about much and I can only presume you'd have to do something similar to mount any kind of credible opposition. That's a…
I'd love to see a good anthropological survey of how Twitter became popular enough that by the time of the Arab Spring it was the default "public communications" app.