Peer behaviors are highly influential in youth decision-making around whether to use marijuana. Social media, as a popular form of peer interaction, has extended peer influence on marijuana use from "in-person" to "on-line" relationships. This is highly concerning, as youth may report their marijuana use on social media as it occurs. The immediacy and possible virality of this type of on-line interaction not only is a source of risk, but also may have utility for monitoring and predicting an episode of marijuana use just before it occurs. This article discusses how in-person and on-line peer interactions contribute to youth marijuana use in order to identify triggering social contexts (e.g., in-person co-location of peers, on-line peer influence) associated with marijuana use. The development of an innovative approach to youth intervention for marijuana use, which combines real-time assessment of social media activity and geographic location of youth and a peer, is described.