I need to do this as well. I use most messengers out there and it serves a purpose to separate them into groups of different friends. The problem is that some of them still use the shitty fb messenger, and the inability to switch off 'last seen' makes them keep track of my online behavior. I have unpredictable, sporadic behavior and just don't log in to fb messenger for days sometimes, and that makes them think I had an accident or something. Is there a hack to disable 'Last Seen' on fb messenger?