Social networking sites (SNS) are credited with organizing protesters in Colombia and Guatemala, and mobilizing voters in the United States. With SNS increasingly used to mobilize collective action, this cross-cultural study surveyed activists in the United States and Latin America to examine how respondents perceived the usefulness and the potential of SNS for activism. This quantitative and qualitative research found that respondents from both regions use SNS to mobilize supporters both online and offline. Whether respondents' activism occurred mostly offline, mostly online, or equally offline and online, they all participated equally in offline activism. Countering previous research doubting the ability of online activism to inspire offline actions, results show respondents believe that online activism translates into offline activism, and that SNS play an important role in contemporary activism. Still, US activists were more likely than those in Latin America to use SNS for activism, or to say their activism occurred mostly online.