[derbOac]

Reddit is (or has been):

1. Relatively easy to create a subforum. 2. Someplace you can post [pseudo]anonymously. 3. Has a critical mass of people. 4. Is accessible to anyone on the web.

There's a subreddit for all sorts of topics, and if there isn't one, it's fairly easy to just start one. You don't have to deal with setting up forum software or paying someone for it, and if you want to you can create a post with a pseudoanonymous throwaway account. It's really easy to come and go, and there's enough moderation that it hasn't in general devolved into a *chan-style environment, but not so much it's alienating.

Pseudoanonymity is maybe underappreciated in social networking I think. It's really a key feature. Once you reach some mainstream critical mass, having that feature really moves things along.

If you could replicate those four things in a decentralized way, it might take off. But there's a chicken-and-egg issue with network effects.