It doesn't seem like you can even log out anymore inside the iOS Messenger app. My wife was without her phone a few weeks ago and needed to message someone via

It doesn't seem like you can even log out anymore inside the iOS Messenger app. My wife was without her phone a few weeks ago and needed to message someone via FB. I could not find the log out button in iOS Messenger, and the only way to do that seems to be logging in to fb.com and hitting a button there which will deactivate your phone session.