You're 100% right. But a good manager also knows that they may be a great employee and, at the very least, grant the interview. A 'wise manager' will be able to separate the chaff that way. But to preclude them solely based on FB posts and totally pass them up for an interview? Now that, my FRiend, is the mark of an unwise manager.
You are assuming that these applicants had a stellar track record. Obviously if an applicant shows a superior work history, certain things will be overlooked (just ask The Dallas Cowboys).