This paper considers Ask.fm, a social networking site where users create profiles and can send each other questions, and analyses aggressive user behavior that may potentially lead to cyber-bullying incidents. We hypothesize that anonymity is a primary cause of such aggressive user behavior and examine how anonymous and non-anonymous users behave in social networking. We collected data from Ask.fm and analyzed questions posted by anonymous and non-anonymous users and answers posted by non-anonymous users. Analysis of the collected data shows that anonymous users exhibit more aggressive behavior than non-anonymous users. Analysis also shows that users become more aggressive in answering aggressive anonymous questions than aggressive non-anonymous questions.