Online social network (OSN) operators are interested in promoting usage among their users, and try a variety of strategies to encourage use. Some recruit celebrities to their site, some allow third parties to develop applications that run on their sites, and all have features intended to encourage use. As important as usage is, there are few studies into what influences users to be active and to remain online. This article studies the lifetime of OSN users, examining the factors that influence lifetime in two OSNs, Twitter and Buzznet. The major contributions of this work are the study of active lifetime, the features and behaviors that encourage activity, and the comparison of active lifetime to passive lifetime.