I don't think it's fair to judge Paper too harshly based upon download metrics -- if I had to guess, Paper was an experiment designed to produce two things: a working prototype of a modernized Facebook UX as "something to point at" for further thinking, and tooling/processes that would allow the wider company to iterate towards that vision in their main app. The idea that Paper was intended to be a popular app, potentially outstripping the downloads, etc of the primary branded Facebook app seems far-fetched.