In two experiments, we investigated predictions of the "collaborative theory of language use" (Clark, 1996) as applied to instant messaging (IM). This theory describes how the presence and absence of different grounding constraints causes people to interact differently across different communicative media (Clark & Brennan, 1991). In Study 1, we document how IM changes as users increase in expertise. In Study 2, we compare adaptations across telephoning and IM with a focus on multitasking. (Contains 1 table.)