Due to their lack of physical interaction, Free and Open Source Software (FOSS) participants form impressions of their teammates largely based on sociotechnical mechanisms including: code commits, code reviews, mailing-lists, and bug comments. These mechanisms may have different effects on peer impression formation. This paper describes a social network analysis of the WikiMedia project to determine which type of interaction has the most favorable characteristics for impressions formation. The results suggest that due to lower centralization, high interactivity, and high degree of interactions between participants, the code review interactions have the most favorable characteristics to support impression formation among FOSS participants.