[projectdelphai]

I would say interaction and a point-system.

For more social oriented places like Reddit, people don't have to stick to one topic but can find a subreddit they like and talk there. They can communicate and share (or endlessly argue) over ideas.

An even bigger reason would be the point system. For all of the sites you mentioned, people earn points and reputation for commenting intelligently or with humor. This is an incentive to stick with a site and rise in the ranks.