Assuming you mean the first part (the validity provision seems "obviously bad" to me), let's lay out some scenarios:
You make your important software dependent on this software. Congrats, facebook can now sue you with impunity because you can't afford to lose these patents.
Worse, the fact that they have given you a grant does not prevent them from suing you, and the second you counterclaim, you lose. Note also that the grant specifically does not cover "the Software in combination with any software or other technology provided by you or a third party." So if you take this stuff and run it in a pipeline, and facebook has a patent on running it in a pipeline, you don't get that patent and they can still sue you.
(Again, in the apache case, you'd only ever lose the patent grant if you sued facebook over the software in question).
Ignoring direct cases, facebook can also use this as leverage during acquisitions that use their software (Nice software, shame if you were to lose rights to it), etc.
There are potentially an infinite number of ways to use this kind of stuff.
Also note this kind of thing has been looked at in the past. There is an entire discussion here: http://mail-archives.apache.org/mod_mbox/archive-license/200...
from when v2 of apache license was being written, etc.
There are reasons the Apache grant has become pretty much the standard - most people and corporations find it a pretty good balance of various concerns (being able to defend yourself in patent lawsuits, etc).
What Facebook has done is entirely one-sided. It benefits Facebook only, and even then, it generally only helps them in situations where they want to sue people. None of this really helps defensively. If they wanted it for defense, they'd write it differently to cover defensive situations.
