Hard to tell how serious he is. I personally think that TC is boring and I'm not a regular reader. But as a reviewer their impartiality is a strength. If people think that negative FB press is sour grapes then the reviews become worthless. Again, not sure if he's kidding but he's treading on dangerous ground.
To the extent that he is not kidding, this is a really bad idea. Facebook obviously have a lot of allure as an employer, get over it. If TC is losing talent to them, that is their problem, not Facebook's. The idea that Facebook should refrain from hiring away any other TC staffers ("or else!") is ridiculous, and doesn't reflect well on Arrington at all.