Facebook may be a big conglomerate these days, and they may not be the cool kid on the block anymore, but that doesn't mean that they're idiots. If anything, they treasure the presence of someone like Carmack all the more, because it's no longer reputionally-free for people like him to come on board. I'm sure that Facebook is appreciative when he blogs on their platform, but it's patently ridiculous to pretend that anyone would -- or could -- force it.
In fact, Facebook has already been forced to pay at least dozens of millions of dollars essentially as a direct consequence of employing Carmack [0], and I'd be willing to bet they'd happily pay (at least) dozens of millions more.
If you want your mind blown even harder, look up some of the paperwork around the Waymo / Uber debacle [1]. SV companies pay big bucks for this type of talent, even the ones you haven't heard of, like Anthony Levandowski.
If the roll of the dice is in your favor, SV stardom is at least as valuable as Hollywood stardom (and, for the most part anyway, you still get to walk around in public unharrassed).
Disclaimer: I have no special connection with any of the companies or people mentioned, I don't know the inside baseball, and this is all conjecture. But at least it's better supported than yours. ;)
[0] https://en.wikipedia.org/wiki/ZeniMax_v._Oculus
[1] https://www.cnbc.com/2017/04/03/waymos-uber-lawsuit-reveals-...
"This fist holds the Ego. This fist holds the Id. I bring you a new existence: the GlobalCoin. I came up with that myself. As Super-Ego."
[not the person you responded to]
In exchange for their patronage, employers to the world's technical stars get to use their name in recruitment and other types of marketing, and they're well-positioned to profit from the star's next great achievement, if indeed one ever occurs. This is very similar to the benefit to a record label or a movie studio in signing a deal with the current one-hit wonder or starlet of the hour.
As in any employment transaction, there can be no true guarantees. None of us can promise that we'll be here tomorrow. But the value of investments in strong personal brands has proven profitable enough over the last several decades that many companies are eager and willing to make them. Entire industries are built around them. I don't think it's going to stop any time soon. cf. the Lindy Effect [0]
There is surely some limit to Facebook's tolerance of a single man's foibles, regardless of star power. I would bet my bottom dollar that "blogged off-Facebook" is nowhere near it.
Otherwise, why would he move away from .plan?