The danger in judging someone according to his or her social networking profile is that the profile may not be true.
No person is under any obligation to put only true facts or photos on their social media profiles. It has been proven countless amounts of time that many people deliberately lie on social media in order to impress others. They put false/very old or heavenly filtered pictures of themselves, they conceal their real age and they lie about their martial status or their job.
This is the reason why one should always make sure the person being judged according to their social networking profile really exists in real life.