From what I can understand from [1][2][3], is the previous patent clause meant that if you and Facebook got into patent litigation, any open source software you used that was written by Facebook would be revoked (opening you up to a license breach). This was scary because Facebook owns a ton of patents and if Zuckerberg woke up on the wrong side of the bed one morning and decided to sue everyone with their portfolio of patents, it would be impossible to countersue if you used any Facebook code. The other posts go into how companies like Google and Facebook use patents as a way of ensuring mutually assured destruction, and if Google had used Facebook code under the license it would be like Google disarming all their nuclear bombs against facebook.

IANAL, but it seems the new license is a bit more GPL-esque in its restrictions by saying if you use this software than you cannot sue Facebook, its subsidiaries, and your parent weapon of choice can't be based off Facebook software (or else you lose the software license). However if Facebook sues you, then you don't lose the license and you are free to counter sue facebook with your nuclear arsenal of patents.

The reason I call it GPL-esque is it seems to introduce restrictions on how you can legally use the software once you've incorporated it (like how the GPL prevents you from not redistributed the source code), the Facebook license seems to prevent you from launching patent claims that are built off Facebook software. My guess is that if enough people used this license it would be difficult to litigate anyone else without revoking some of your software licenses.

Finally, I never took a law class a day in my life and the closest domain knowledge is an intro to econ class I took in freshman year of college so I could 100% wrong.

[1]https://news.ycombinator.com/item?id=9271246 [2]https://news.ycombinator.com/item?id=9111849 [3]https://news.ycombinator.com/item?id=9113515