Carmack appreciates impressive technologies when he sees them and has always humbly voiced his support for them. Back in the dark ages he called Ken Silverman, the developer of Duke3D's Build engine -- the supposedly direct competitor of Quake at one point -- the most talented graphics programmers that he knew besides himself. He had similar praise of Oculus VR before he joined the crew.
No, he's isn't a saint in any benevolent sense, but when it comes to commentary on developing technologies, I tend to trust him -- personal disdain for Facebook's sociocommercial business model aside.
Also, Carmack's next Twitter post directly communicates that he's been avoiding creating a Facebook profile up until this point. So perhaps his admiration of the company on a social level is not as strong as his respect for them on a technological infrastructural level.