So, it looks like we're going to have to start killing a massive amount of people pretty soon so that we don't hit that fateful number. I'm sure that the second we do hit 6,666,666,666, the entire world will blow up, killing everyone in it. The best course of action is probably to start WW3, that should kill off enough population to prevent us from reaching 6,666,666,666 for quite some time.