Like it or not, when someone is a public figure, their posts on Facebook get LOTS of views. A larger percentage of internet users ONLY read Facebook these days.
Carmack's profile has almost 10,000 followers and several hundred 'friends' each of which will have been pinged about the new post immediately. Even a popular blog won't be able to do that, that instantly. So you can see the attraction.
Also, on Facebook there is the concept of 'Notes' which are better for long form writing and display in their own large overlay div. Not sure why Carmack didn't use it.
I am not a fan of Facebook, but from a technical point of view the platform does have its merits.
Just because someone has worked for Facebook, or currently works for Facebook, doesn't make posting these sorts of articles on Facebook ideal.
Even Facebook knows this... which presumably is why they chose to use a traditional blog style site for their... blog posts[1].
[1] https://developers.facebook.com/blog/