This article contributes to the geographical understanding of how mobile online presence enabled by smartphones transforms human spatial practices; that is, people's everyday routines and experiences in time and space. Contrasting a mainstream discourse concentrating on the autonomy and flexibility of ubiquitous (anywhere, anytime) use of social media, we examine new and mounting constraints on user agency. Building on timegeographic theory, we advance novel insights into the virtualities of young people's social lives and how they are materialized in the physical world. Critically, we rework the classical time-geographic conceptions of bundling, constraints, rhythms, and pockets of local order; draw on the emerging literature on smartphone usage; and use illustrative examples from interviews with young people. We suggest a set of general and profound changes in everyday life and sociality due to pervasive and perpetual mediated presence of friends: (1) the emergence of new coupling constraints and the recoupling of social interaction; (2) the changing rhythms of social interaction due to mediated bundles of sociality becoming more frequent and insistent; (3) the shifting nature of the streaming background of online contacts, which are becoming more active, intervening in, and intruding on ongoing foreground activities of everyday life; and (4) the reordering of foreground activity as well as colocated and mediated presences, centering on processes of interweaving, congestion and ambivalence, and colocated absence.