How do you think of Reddit in that framework? Reddit itself is explicitly a tool for creating, basically, community sites. How closely does it fit with your vision of a tool for crafting social networks? When do you predict it will fail?
Reddit revolves around people finding interesting content and providing it to others to be consumed with some commentary on the side.
I'm not sure how they're comparable. Sure, reddit has a community aspect, but it's really just a side dish compared to the content (links).
Many of the subreddits (e.g. the ones dedicated to cities or schools) are far more about the community than the content.
Maybe if you stick to the default subreddits that are composed entirely of memes.
The main reason that I use Reddit is for the discussion; I highly, highly doubt that you're going to find /r/askscience-calibre conversations on Facebook.