Friend's List Bug?
The friend’s list isn’t updated in real time. When you attempt to whisper, the system will refresh your list first, which is why you sometimes see someone appear to suddenly go offline.
The refresh delay varies considerably in my experience, sometimes it’s negligible. I think that’s more common when I’m in WvW or in a world boss event (massive zergs nearby in both cases), but I haven’t done any rigorous testing.
John Smith: “you should kill monsters, because killing monsters is awesome.”
It just seemed targeted is all. I would give it a few minutes and they wouldn’t go offline, but as soon as I tried I would get a message stating the person was no longer online. Then a few seconds later would reappear online.