This is the most private mode of communication. There are some great resources like writing activities and tips, craft instructions that are available on Tumblr and you don't have to have a blog to go directly to it. Or at least a parent should teach FB etiquette to kids like on smstrackers.
Private Chat Lobby
You correctly point out that no amount of monitoring, including using software, can cover all the bases. We alerted the local police and reported the incident -which they fully supported. I had zero idea that this many apps existed and parents simply must be informed!