David McCandless explains:
In this diagram, each level in this hierarchy trumps the next. So, if you get a new msg on Facebook, but your landline rings, you’ll take the landline call. You might have a spasmodic moment of ‘uh? wadd I do’. But, usually, you’ll take the call.
Strangely enough it’s missing AIM, MSN, GTalk and other IMs… unless texting has replaced that and I’m terribly uncool not to know it?!