Last year for the first time I found some of my students reporting that they had stopped using Facebook/Bebo/similar and even deleted their accounts - and over the past few years have consistently found that many students do not use the sites.
The Guardian reports on findings from Ofcom that teens are now spending less time on social networking sites http://www.guardian.co.uk/media/2009/aug/06/young-abandon-social-ne...
"Although their love of being online shows no sign of abating, the percentage of 15- to 24-year-olds who have a profile on a social networking site has dropped for the first time – from 55% at the start of last year to 50% this year. In contrast, 46% of 25- to 34-year-olds are now regularly checking up on sites such as Facebook compared with 40% last year."
The same report shows high take up in the use of social networking by younger teens though.
Personally I suspect one issue will be the competition for different forms of social networking competing for attention - gamertags, xbox live, facebook, etc etc... and an adoption curve where a new platform gains enthusiastic converts who then slowly and over time drift away to the next platform (or fall back to their last social network...)