From the very first studies on social uses of computer communication systems, researchers were already analyzing the new kinds of social ties that were emerging. Today, the success of social networking sites has brought renewed interest in this topic. Possibilities for creating wide-ranging relational networks whose members may move in circles that are very far removed from ordinary social life have given rise in particular to studies on how these networks of “friends” come into being, on their social morphology, their topographical structure and, more rarely, on the motivations and social significance of online engagement, which goes hand in hand with other people’s motivations. Looking into the role of online communication in the creation of social ties, research in the UK and USA has focused in particular on approaches based on theories of social capital. This article describes the main results of these studies and shows some of their limitations. We also consider several avenues for further research, from a more critical perspective, on the question of social ties on social networking sites.