Humanity will most likely always be on Earth (at least in some form of settlement, whether it be a Star command base, a ground for energy reactors etc.) A lack of resources to sustain a total population would be the first reason why we would leave in the first place, but the resources here are already enough to sustain 10's of billions of more people. Once our population exceeds that however, I'd wager a guess that we'd split into two sort of groups: those that stay here, and those that choose to settle elsewhere. We're very early on in our total development as a species, and a Moon or Mars colony would probably be our first real ventures in working that idea. Past that into the future though, Earth may yet become a remnant of the past, a forgotten relic, like many old English villages that still operate today; quaint and outdated, but still there to look back from where we've come.