There are lots of Facebook groups I like. No one is talking about my neighborhood on HN! But besides interesting local groups, there are amazing niche technical groups on Facebook that would crossover with HN. A few I find useful:

Apple ][ Enthusiasts: https://www.facebook.com/groups/5251478676/

BMW i3 Worldwide: https://www.facebook.com/groups/BMWi3/

Monoprice Mini (3d printer group) https://www.facebook.com/groups/710952782398723/

Commodore Amiga: https://www.facebook.com/groups/CommodoreAmiga/

I tried to create a group on cryptography (in order to fill my newsfeed with interesting posts) buy nobody seemed to be interested. If anyone here is I could post the link.

My buddies from highschool and I used to have one for posting goofy internet stuff, but it's been pretty dead for years.

I like many groups, although they don't have better discourse than HN.