But how intimate is it, exactly, when you count up all the participants: yourself, your phone, your data carrier, the person on the other end of the line, their phone, their data carrier, the corporations who run the servers that move your messages to and fro. And probably a government or two. It's more like a party line.

The social networking giants promise secure backchannels, but that's not always the case. Even if your ex isn't reading your Facebook messages, Facebook might be. The company was sued in 2014 for allegations of data mining personal messages in order to serve more precise ads. Last year, Google announced it's going to start mining your inbox activity to better target its ads to you and your network of friends.