Signal is the world’s most widely used truly private messaging app, and our cryptographic technologies provide extra layers of privacy beyond the Signal app itself. Since launching in 2013, the Signal Protocol—our end-to-end encryption technology—has become the de facto standard for private commu...
What the other responders said (there are great clients out there, that fit mainstream and niche needs).
Also, it is not a problem of “federated protocols” per se, but of community-led projects. On the downside it may lack consistence and direction, but on the upside you can step in and contribute feedback, tests, documentation, and why not, code :)