Your explanation really doesn't jibe with the explanation given in Facebook's post. If what you say is true, why do they make a point about this patent grant protecting them against patent trolls? There's something tricky about this that I can't figure out, but I really have a hard-time believing they're acting in good faith here.
If we use the "minimalist" interpretation like you and others propose here, then I'm stumped by Facebook would bother. It's such a tiny, ultra-specific advantage for Facebook. Why throw this gas on the fire for such a minimal advantage? Just leave it alone.
I suspect this must be coming from Zuckerberg. He's pissed off that some patent troll, somewhere, was using Facebook open source, and he issued an edict. That's the only way this makes any sense at all.
