Mobile devices are common communication tools, which deeply affect social engagement practices. Smart-mobile devices move beyond voice and textual communication: they enable ubiquitous online connectivity, and bring changes to mediated social interaction. In this paper we bring the results of a study of the meditated social practices of students who use smart mobile devices on a university campus and beyond it. While the common premise is that smart mobile devices enable continuous collaborative interaction, our study shows this interaction is limited than previously believed. Two distinct factors were found to affect mobile interaction: sociability and usability. While sociability entices users to engage in continuous mobile-mediated interaction, usability issues encumber the full embracement of mobile-social applications. The tension between the two creates a new form of interaction - "intermittent participation" - in which users are constantly attuned to absorb notifications and updates, but rarely respond to them, unless a response is absolutely necessary.