On the East Coast, great whites don't really congregate (except off of Cape Cod), because there are few large feeding grounds like the seal and sea lion gathering spots on the West Coast. The sharks also don't have to breathe at the surface like whales do, and so they can be very hard to spot by humans. They are often solitary and rare, and are very spread out. In the East, they tend to hang out in Florida in the winter and migrate north during the spring, even up to Canada.
Great white shark populations can decline quickly because they grow slowly, don't reach sexual maturity until late and females have to have their eggs fertilized internally, so they have few offspring. The population increase is very good news indeed, since previous studies estimated that their numbers were much lower - up to ten times lower off of California, for example.