The new open source patent grant of Facebook
See Updating Our Open Source Patent Grant
So today [10 April 2015] we're pushing a new version of the grant. Its main significant improvement is that it is clearer with regard to what we mean by 'affiliates' and 'assertions' and the circumstances under which the grant terminates, all of which had caused confusion. You can see the full text of the new grant on our osquery project, for example.
As of today, this version will be used on all new BSD-licensed Facebook open source projects. What's more, we are currently updating all the projects that used the previous version, and they can also be used under these revised conditions as soon as the diffs land.