I think the argument is that the grant is asymmetric. GPL is "I open up, you open up". The retaliation clause Facebook uses is "you don't get to sue us, but we may sue you".
That's an interesting argument! (But the article didn't make it..)
Couldn't you argue that the GPL says "you don't get to keep your changes private, but I do", and so is asymmetric too? It seems normal to me for the rightsholder to retain more rights than the grantee.
You can keep GPL changes private if you don't publish it, and the same applies to the original author. If you feed back changes, you're mutually bound.
But the problem with Facebook's license is not the copyright, it's the patent grant.
