IMHO, Matrix currently has the best shot at becoming the standard for the open internet.
Those who value the freedom of choice should push for Matrix before Signal becomes the de-facto standard and is acquired by one of the tech giants looking to lock down control of communication.
Matrix is too dependent on it's only vendor in existence, their only server in existence has performance issues. Also, a monolith standard is hardly viable for federated networks, where all nodes can't upgrade all at once.
Ah, I did not realize you were referring to the software. In that case, there is also Construct[^1]. There are other implementations in the works, though progress is somewhat slow.
Synapse is much better with regards to resource usage these days, though. The RSS of my instance is 355M right now and the CPU usage is hovering around 0-10% (15-min server load ~0.5).
I wouldn't use keybase where I need reliability due to their vague TOS (that came into effect around the beginning of this year): They can ban you from their service if they deem your actions outside their platform unacceptable.
Interesting, I had to check their ToS and found this which seems a bit vague:
"[...]use the Services to store or transmit any inappropriate content, such as content that: (i) contains unlawful, defamatory, threatening, abusive, libelous or otherwise objectionable material of any kind or nature" [1]
Those who value the freedom of choice should push for Matrix before Signal becomes the de-facto standard and is acquired by one of the tech giants looking to lock down control of communication.