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.
Public announcements of resignation have two impacts: on fellow employees, and on the employer. Fellow employees may be waiting for validation of a sinking feeling that they're in the wrong place, before they quit themselves. When the employer sees numerous conscience quits in response to a bad action, they have an opportunity to reflect and change.
Real good deeds aren't announced.