You can look at how email is monetized for some ideas. Everything from ad infested free client to pay service to freebie with the purchase of your internet connection.
I suppose you could have an 'Apollo' style app providing a nice UI for power users with a subscription cost. I could see it being a solid revenue stream for small teams/indie makers. Difficult to get to the kind of scale needed for advertising to be particularly profitable unless we get some kind of federated facebook ads platform...