They are not successful because Signal has not yet done anything particularly egregious. If they do, I believe that a fork would quickly gain popularity.
>I know of many people on both of the platforms I mentioned. Collaboration between all of these platforms is frequent and they drive improvements in each other.
We've both given anecdotes; user counts would be more conclusive. I've found +1 million [0] for Mastodon. Do you know how many users GNU Social and Pleroma have? I can't find these numbers from a quick Google search, but they might be somewhere.
[https://en.wikipedia.org/wiki/Mastodon_(software)#Adoption]