When users interact with one another on social media sites, the volume and frequency of their communication can shift over time, as their interaction strengthens or weakens. We study the interplay of several competing factors in the maintainance of such links, developing a methodology that can begin to separate out the effects of several distinct social forces. In particular, if two users develop mutual relationships to third parties, this can exert a complex effect on the level of interaction between the two users — it has the potential to strengthen their relationship, through processes related to triadic closure, but it can also weaken their relationship, by drawing their communication away from one another and toward these newly formed connections. We analyze the interplay of these competing forces and relate the underlying issues to classical principles in sociology — specifically, the theories of balance, exchange, and betweenness. In the course of our analysis, we also provide novel approaches for dealing with a common methodological problem in studying ties on social media sites: the tremendous volatility of these ties over time makes it hard to compare one's results to simple baselines that assume static or stable ties, and hence we must develop a set of more complex baselines that takes this temporal behavior into account.