What is the life span of a social network?

I suppose social networks (of the Internet kind) haven’t been around that long (relatively speaking), but I wonder how long they last. There appear to be gazillions of them now. Like dandelions, they have spread everywhere.

Are they like news stories? Hugely popular till the next one comes along? Facebook, Twitter, MySpace … (Is MySpace still big? I could never get into that one.)

I imagine the life cycle has two aspects: public and private. A network may be popular but an individual may drop out. However, when enough individuals drop out the public popularity drops.

What about social networks generally? Will they lose popularity when the next big thing comes along (whatever that may be – social networks 2.0?)

Part of the reason for wondering about these things is the sense I get that a great deal of what becomes popular on the Internet (and even those things that aren’t so popular) are dependent on the “passing fancy” factor. Something looks kind of neat, a person gets involved, then loses interest.

How much is function a factor? LinkedIn has always struck me as one of the most practical of networks, in theory. But I rarely use it. I use Facebook quite a bit but that is largely because of the number of friends who are also on it. But other than the contact factor, I don’t really have much use for it.

What (if anything) must networks like Facebook need to do to sustain beyond a 3 to 5 year cycle? If my suspicion is correct that the majority of Internet networks, sites etc. have a fairly brief life cycle of success (they may linger for years but for all intents and purposes be kaput), does it make sense to invest money, time and effort into them?

Just wonderin’ …