The patents clause does not restrict to software pattents, it applies to any patent. If I had any "normal" patents, I would stay far away from any open source project with this patent license (also I'm not a lawyer, this is not legal advice). I would never allow Facebook (or any company for that matter) to limit my legal rights. I understand that they need to protect themselves, but not at the cost of the damage that they are doing to the open source community.