Twitter is increasingly accepted as an important educational technology and has been shown to serve a range of purposes. In fact, this variety suggests that Twitter has the potential to serve as a foundational technology: one capable of supporting teachers' learning across multiple formal and informal contexts. To explore this possibility, we examined the purposes that Twitter serves in one educational technology graduate program. We collected over 9000 tweets containing any of 12 program-related hashtags and coded a sample of them to describe the purposes they served. This resulted in six themes: contribute to disciplinary conversation, engage with disciplinary conversation, build community, make connections with other communities, ask for and provide support, and unclear or irrelevant purpose. These themes--and the varied contexts they were associated with--suggest that Twitter serves as a foundational technology in this program and has the potential to do so in other educational communities.