I think the key bit is Notwithstanding the foregoing, if Facebook or any of its subsidiaries or corporate affiliates files a lawsuit alleging patent infringemen

I think the key bit is Notwithstanding the foregoing, if Facebook or any of its subsidiaries or corporate affiliates files a lawsuit alleging patent infringement against you in the first instance, and you respond by filing a patent infringement counterclaim in that lawsuit against that party that is unrelated to the Software, the license granted hereunder will not terminate under section (i) of this paragraph due to such counterclaim. In other words, it's an explicitly defensive capability they are reserving - to revoke your patent grant iff you are the one to initiate a patent suit against them. If every open source project included such a statement, it could potentially do a lot to end software patent wars.