I don't do Facebook so can't comment. I'm not sure Twitter will give you what you want, it's good for news, for following corporate stuff, for collosally niche weird stuff, for gawping at political loons but it doesn't (for me) have that community/shared history vibe of a forum.
Reddit can be good for some subjects and areas.
Personally, a lot of my forum time has migrated to pistonheads, it's essentially the blokes version of Mumsnet. High levels of traffic, lots of regulars/tropes/themes, threads on everything and anything <checks recent posts> such as the Beiruit explosion, the 1700 tons of high explosives 50 feet below the surface of the Thames, cars you didn't know existed, photography, DIY, lawn care, food, home brew and just about everything else. Full of grumpy old men so I fit right in. The news and politics threads are usually somewhat to the right of Genghis Khan and discussions on Gypsies and Travellers are horrendous but I just avoid those threads and enjoy the rest.
That sounds interesting Mr Jeans, I'll check it out.
I've been enjoying Pistonheads. Haven't joined but lurked a lot.
I think I'm going to open a Facebook account though. There are a couple of groups there I want to investigate, (Maxi scooter owners & motorhome & campers) but they are members only.