Comments moved to https://news.ycombinator.com/item?id=9356508.

Blog post about the change here: https://code.facebook.com/posts/1639473982937255/updating-ou...

Relevant snippet:

Any inadvertent doubts [the previous Facebook-specific grant to the BSD license] introduces detract from what we want our program to be about: world-class communities and world-class software.

So today 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.