Obviously nothing stays around forever. But there is a big difference between a trendy app that everyone uses for a couple years and then stops, and Facebook, which many people don't use on a daily basis but still keep it around because its usefulness outweighs the effort required to keep your account. I have friends who have deleted their Facebook accounts, only to come back a month later with a very basic account just because they need to keep in touch with some circle of friends that they otherwise wouldn't be able to.