Well.. For social things they might be.
I mean, lots of folks flock to Facebook just because: 'I'm on there already to see what aunty Agatha is up to, so if I can see what's up in that-and-that community without th hassle to actually go to a forum, I'm all for that'.
Discord is more of a newer incarnation of the older IRC (atleast, that's what I gathered when I used it for a few sessions). I recall I did make an account, but found it unwieldy to work with, so I've not really gone back to it. I still drop by IRC every now and then tho
I do recall there is still a #fukufics channel on one of the IRC networks too. Recall I had that entered into the config of the client, but I'd have to look that up if applicable. Don't have that info readily at the forefront of my thoughts.
Forums I feel however do fill a specific niche.
Where a chat helps you directly, it's rarely possible to go back to any chatlogs years past to review what was said unless you logged the chat yourself. And even then it disallows others from finding any answers you were given to possibly solve their issue...
Facebook is somewhat of the same... But while the search function is available, finding something specific often is tedious, and innundates you with results that seem to have nothing to do with your query. So actually finding something there gives you too many results to sift through. Facebook has just wayto broad a dataset (everyone has different interests) to make zero-ing in on what you are interested in a hassle.
A forum has a specific topic (and a branch of the forum may even focus on specific topics), and thus searching on a forum would already majorly restrict all results to just that topic. That cuts down on the search-result clutter tremendously. It leaves searchable results which (as long as the forum remains around, of or the WayBack Machine has a cache entry of it) remain available to people. The only restriction in searching could be you'd have to be a member of the forum, since some forums don't allow anonymous browsing/searching.
And like I said... I frequent this forum, a couple of Mustang boards (just to read up, keep in contact, and occassionally ask questions about things to sort problems I'm having with mine. Beyond that some IT boards (SpiceWorks comes to mind). Tho the IT ones are more to pick the minds of others in regards to issues I need a fresh perspective on or just plain can't figure out on my own. I'm still very much a Forum guy.