We should have text messaging (instant messaging) in Loop. So we can use it instead of Audio/Video, through the same channel. Or in combination with - for off band message during an audio/video call.
This feature should be in all the clients we have: Desktop, Mobile, FirefoxOS. For FxOS, having it into the messaging (SMS) app would be even better.
Bug 1108892 is implementing text-chat for within conversations. Persistent IM (i.e. not both folks being online) is a longer term item that we're thinking about now but have no etas as yet.