I mean, people aren't really keeping up with the top posts 3 weeks ago on HN, reddit, or twitter. That's why reposts are so common.
The first two are easily indexed by google, so that's one advantage. Twitter has a site-wide search. Discord doesn't (yet?) have such a thing, but I think the focus being private communities might keep that from happening... unless they get bought out and some MBA says "how can we better get clicks?"
USP, Unique Selling Proposition, a term I'm shoehorning for wan of a better one.
Different USPs, or pseudo USP as it's a weak U, more a network effect.. The USP of Facebook being closing a circle Facebook controls around you. Of Reddit being indexable/discoverable communities/interests, of Stackoverflow focused, transactional answers. Of Twitter being bullied into change from their earlier premise to serving bigmedia noise. Of Discord wasting users time for the reward of 'being part of..', that's their USP and Discord would die without the waste time through repetition that holds membership (for now).
The first two are easily indexed by google, so that's one advantage. Twitter has a site-wide search. Discord doesn't (yet?) have such a thing, but I think the focus being private communities might keep that from happening... unless they get bought out and some MBA says "how can we better get clicks?"