I strongly disagree. Quitting your job is just about the most disruptive thing you can do in terms of the effect it has on your own life. Some folks might have an easier time finding another job than others, but that doesn't change the fact that he's actively choosing to stop creating value for an organization that his morals don't align with.

And additionally, quitting your job on moral grounds is _absolutely_ something you should post about. He made a personal post on Facebook, as a current Facebook employee, noting that he's leaving. This will be seen by his coworkers, and as the very presence on HN demonstrates, the public at large. Virtue signaling is Old Navy changing their profile picture to a rainbow flag during Pride month, or an NFL team posting a black rectangle on Instagram to support BLM. Virtue signaling is _not_ quitting your job to protest your employer's ethical failings, or writing content that would violate a non-disparagement agreement.