Social Fixer (disclaimer: I am the author) has been around a lot longer than FBPurity, and was the original Facebook extension (originally called Better Facebook). The author of FBP was kicked off Facebook because of his questionable practices and breaking of rules. It's used by more people (at the moment) because he maintains it more actively than I can right now, and he shamelessly promotes it. Over the years, features that appear in Social Fixer mysteriously appear shortly after in FB Purity. That's some background.