I didn't think of that.

3 minutes does seem like the kind of time an engineer would code into their app.

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.