1) Well we have largely left natural selection behind. But in order to us to have "left Dunbar's number" as well we would have needed natural selection as the pressure to do it. You need a selective mechanism if that makes sense. You can't rewire the human brain to be able to remember thousands of relationships at once unless their is a survival advantage over many generations to doing so.

2) Our neocortex is larger than any other primate, and we can exist in groups and remember more social relationships than any other primate. Chimpanzees for example roam around during the day in parties of 8-10 and exist in larger groups of up to 50 individuals. And chimps live in large groups for primates. Gelada baboons are one of the only other species that exist in groups of over 100 individuals, and that may be because of their unique ecological circumstances (not a coincidence that they exist in a savanna landscape).

3) I think that is a fair argument. As I said, the Dunbar number is a hypothesis. I personally feel as though there is more evidence supporting it than against it, so overall I find it conceptually useful. But future research may disprove it. I guess as a counterpoint to your point re: Facebook, I would say that that is an unfair comparison because you are having a computer store your friends for you. If you didn't have Facebook you wouldn't remember details about all those 500 individuals. However, that is a very small example of how we are using technology to increase our intelligence. In the past decades (and in the coming decades) we will be expanding our neocortex. I am smarter than I would have been if I lived 30 years ago. Google, Wikipedia, etc. make me smarter. They allow me to have access to all human knowledge. In the case of Facebook, they allow me to push my brain the capacity of social knowledge. This might be getting too far into it, but in the future we will use nanotechnology to actually expand the neocortex's abilities. At that point I would expect Dunbar's number to explode to perhaps include the entire human population. #TheSingularity