alt Hacker NewsXMPP and IRC are great and all but a massive part of what people use Discord for is group voice calls with screen-sharing. I'm not sure what the alternative is for that. TeamSpeak is the closest I can think of but it's not a 1:1 replacement for a number of reasons.
We use Jitsi for voice/video calls + screen sharing (but I realise you may be talking about all-in-one alternatives)