Honestly, public & private FB groups. Groups dedicated around specific subjects (mostly related to my career) have been some of the most influential places for learning, networking, building relationships that I've been able to call on later.

FB groups are king because:

1. Everyone is already there. 2. It's tied to your real identity (for the most part).

Other areas to learn/network and ask dumb/advanced questions exist, but to "community" vibe in FB groups is unrivaled.

My FB feed is almost nothing but groups posts. I've hidden/unfriended most people I don't care about and no one I've kept really posts to their actual pages much these days anyway.

I really wish Groups was a separate app. I'd keep that and messenger.

I realize my privacy on fb, and likely around the whole web is the tradeoff for this but unfortunately that's what's required to stay on. The advancements I've made in my career and personal life directly & indirectly due to the FB groups I'm in are definitely worth it, and likely would not have happened without the existence of FB groups.