"If you have just a slight change in the ordering, particularly something that might bubble up from eighth to third or fourth on the screen where you're far more likely to see it, then just that simple change in ordering might have a huge influence on what you click on".

BBC: Click listeners test 'filter bubble'

You can reply right there without having to leave Google Maps or whatever app you happen to be using, even move the bubble around so it's not blocking a certain part of the screen.

CNN: Review: A closer look at Facebook Home