Slack is just irc, Dropbox just nfs, Google just grep, Craigslist just newspaper classifieds, amazon just Walmart, Netflix just charter, hacker news just coffee shops and bars, and on and on, what's your point
I like reddit because it's a bunch of millennials and has been from the start. My old manager, he talks all glowingly about the haydays of usenet. He's a gen-Xer. Given the equivalency of social networks features, the most important thing is just going to be the identity of the users consuming them.