IP + channel + date and time of messages + payload size. And if you use the same server for all your contacts (which is what everybody does) then they can see the group of channels you talk to. If both sides use the same server (which is common) the server can map IP to IP, geolocate the pair. Track IP over time and you can isolate work/home locations and from there you can know who the person is, with good probability of sucess.