I think from what we've seen, there are plenty of survivors left out there. From the Map a relatively small area of the US had 4 big clusters of survivors(The Tower, The other tower, the mallers, and the colony). And that is right around a ground zero in one of the most populated areas in the US. I'm sure there is plenty of other Factions out there, probably a lot better areas to be in. My favorite thought is of a US Naval ship, nuclear powered, that are stocked for long term deployment, although hitting the coasts as needed.

As for the future of the Factions we are familiar with... I think they are meal time. I think the Mallers and the Colony are going to go down, go down soon and go down hard. The zombies are running out of corpses, and getting hungry like those from the island. They are going to organize and hit the pockets of survivors, maybe even roaming or smaller organized groups of zombies. This is going to put the Tower in a desperate situation in that they have to get out, and not in as prepared a manner as they would like.