With the growing sophistication of social media, virality of online content has become an indicator of online message effectiveness. We argue for a comprehensive definition that extends virality to social networking and microblogging sites, by emphasizing users’ behaviors beyond shear access and viewership. Across two studies, we investigate viral behavioral intentions (VBIs) toward pro-social messages shared on Facebook and Twitter. We further explore how motivations and uses of Facebook and Twitter predict VBIs toward messages shared on these websites.