I go to Reddit and HN for technical topics I have an interest in. Mostly Machine Learning on Reddit.

For more political and personal topics, Facebook is far better. Mostly because there are far more Facebook groups than there are subreddits, so in a controversial topic you tend to get vicious, abusive individuals taking control of a given category on Reddit. Because there are a lot of these folks out there and there's no objective way to remove them. On Facebook, you can create a group on topic X, run it sanely and gradually get credibility and get people migrating from the less-sane groups. If there's a way to do this on reddit once some dubious character has claimed a given topic, I'd like to known.

And for really small groups, Dischord seems best.