I find myself lately re-entering everyone I know into the system every year or two; I remember Six Degrees, Friendster, Linkedin, (I skipped MySpace -- I'm too old,) Facebook, Dopplr, Flickr, and so on. Brad Fitzpatrick seems to agree that this is an annoying waste of time, and says so in his thoughts on the social graph.
Most social systems never forget anyone. Given that recent behavior appears to send friend requests to anyone you've ever met even briefly, I find my contacts list ends up filled with people I don't really know. In many systems, removing someone from your list is either buried or simply impossible. Further, since these systems make implicit relationship information explicit, deleting someone becomes a loud signal. In real life you would merely back off a bit, but the systems only allow you to express a binary sort of relationship.
Therefore, switching networks becomes a way to regularly cleanse your contact list. There is evidence that younger internet users regularly start new instant messaging IDs; this likely serves a similar purpose.
So perhaps frequent switching is less a function of fashion but instead a coping mechanism to deal with the mismatch between reality and software.