>So does that mean that the spiky periods are times when people are online the whole time?

I'd be inclined to think so, but I don't work for Facebook, there may be an entirely different reason :)

Edit: or maybe, maaaybe, it's the point of time when people fall asleep over their mobile - they've stopped interacting with the app, it still sends a few keep-alive requests, and then logs itself out.

Could it be an unusual client, like a microsoft phone or something?