Unlike traditional media, social media rely on high levels of consumer engagement, involvement, co-creation and propagation. By the very act of forwarding a viral message, there is an implicit endorsement of the content and the credibility of the message is enhanced. However, the fundamental factors affecting a recipient's decision to forward a viral message have received scant attention. To address this gap, a study was undertaken using three YouTube videos as exemplars of viral peer-to-peer stimuli often shared within digital social networks. Findings suggest that sender involvement and the amount of online communication across the tie are the most critical factors influencing propagation propensity. Tie strength, while still significant, was found to be less important, while knowledge of sharing YouTube videos online had no significant impact. This study provides the first empirical evidence regarding the factors affecting the forwarding of digital content across social networks. Copyright © 2011 John Wiley & Sons, Ltd.