I've seen UseNet die, so I'm not so sure. What Reddit does (and UseNet before it) is really niche. There aren't as many people on Reddit as there are on Facebook and Reddit isn't as entangled with people's 'real' life as much as social media is.
Don't get me wrong, I like Reddit. Before it I used Usenet, which died and after Usenet I was active on forums (most of those also seem to be gone now). I really prefer those topic based social media to the other user-based social media. But I've seen enough of them die before.