A social networking system detects spam in comments posted by users of the social networking system. A spam prevention module of the social networking system retrieves content signals associated with a posted comment and analyzes the content signals to determine whether the posted comment contains spam content. The spam prevention module augments the content signal analysis by analyzing social signals of the posted comment. Based on the content signal analysis and the social signal analysis, the spam prevention module determines whether the posted comment is spam and takes remedial actions on detected spam, including blocking the comment and educating the user who posted the comment.