I think one of the best ways to do this, is to keep the social system smaller once it reaches critical mass. It sounds counter-intuitive, but hear me out.

New users that join once critical mass has been reached, will be able to create their own graph and enjoy the social network in question equally to when a person joined it 4 years ago (given the site is still the same in terms of functionality). However, the person that joined 4 years will become bogged by an ever increasing graph of connections to pages, friends, groups, history etc that eventually undermine what made the site relevant to them in the first place (ie Dalton's experience with Orkut).

So, just as in the real life, social relationships come and ago, a natural decay has to happen that allows a person's graph to remain relavent. Currently Facebook (and to a certain Google+) are doing it right. They filter. Heavily. If you don't interact with the people from 4 years ago, you just don't see them anymore on it.