Because we are (almost) never in the same room to communicate, and are distributed across time zones and continents, team communication is a critical piece of how our team functions.
Depending on the subject matter or the need for synchronous vs. asynchronous coomincation, we use different tools.
Generally, for real-time communication and team meetings we use Slack calls and Google hangouts. For asynchronous communication, we use Slack and if the issue warrants tracking to completion or needs the input of the development team we use GitHub.
Email is generally reserved for HR-related requests and reimbursements.