In preparation for the master we, Sarah and I, have been using slack as our main communications platform. This, as it turns out, has been incredibly helpful, due to the app integration.
Also I didn't want to use facebook chat. Not for "official" business.
Basically, I've set it up so that Trello, BitBucket and Google Calendar are all linked to Slack, and send activity logs to different channels. This makes sure that both of us know exactly what is going on at all times, and that makes communication a lot easier. No need to ask someone to do something, or when a deadline is due, just go to the appropriate channel and see if it's there. Whenever something happens I get a notification.
For example: Sarah pushes an update to BitBucket and moves a task from "To Do" to "In Progress" on Trello. I get immediate notifications from slack that these things are happening, and, without exchanging a word, I know Sarah has finished something and started work on another.
However, it's not just about real-time updates, but also keeping a log of things. This is so that we, in a years time, can look back and show exactly what we did at which point in time, what we discussed during meetings, which builds were shared,... pretty much everting. This can be used during presentations or to show people who would be interested in our creative process.