Shoutbox

It's hard to say.  I believe everyone saying it doesn't work, but I recently reformatted my computer (so I have no cached data) and I have contacts showing as both having me and not having me.  The overall picture is also believable -- the smaller number of people showing as not having me are people I don't talk to often on Messenger and it wouldn't surprise me.  It looks like it works, but I have no way to know how consistent it is.

Edit: I just asked someone I primarily talk to elsewhere, who was showing as not having me on Messenger.  He does, so I guess it doesn't work.  I am curious what the logic is though, because all my frequent Messenger contacts still show that they have me.