Prosody is also a great server with a ton of functionality.
For the tech crowd I think Signal was just very enticing as it was easy to convert non techies with smart phones. That’s the discovery arguement, but I find that point moot since a properly configured setup should allow one to use the same address as ones email address for XMPP (much like gtalk). Now signal claimed to have social graph anonimity, but for the longest time that was not true at all for a state sponsored adversary (it has technically improved but I’m not 100% sure that is true in practice).