What is the most popular social network for developers?

I would actually say Github :) From the list you gave Reddit and Twitter should be the 2 best.

There are many subreddits that are specifically for developers and I imagine the majority of developers have a LinkedIn profile, but also Hacker News (https://news.ycombinator.com/), Discord, like Reddit has developer communities. Depends on what you're after really.

Reddit

I believe the most popular social network for developers is "Reddit".

Reddit, unequivocally.

Medium!