There is an ongoing debate in the blogoshphere on the permanence of the Social Networking phenomenon. Is it a fad or is it here to stay? The debate was fired off by Ryan Carson in his article "Why I dont use social software". In his blog on the Wired forum, Michael Calore provides a brief summary of this debate.
I believe that the community-based activities that underlie the social networking sites (the so-called Web 2.0 platforms) are here to stay. While the nature and the intensity of these activities might change or evolve over time (based on the enabling tools and technologies), the basic human motivations that drive such activites are not likely to change - whether those motivations are simple hedonic factors (e.g. having some fun) or more complex socio-psychological factors (e.g. the need to influence others through one's ideas; the need to be regarded as an expert, the need to belong to a community, etc.).
Research done by people like Yochai Benkler, Josh Lerner, etc. have shown the importance of these motivating factors. In my own research work on Online Customer Communities (or Virtual Customer Environments), I have found considerable evidence for both these sets of factors. In addition, I have also found that the 'opportunity to learn' is another critical motivating factor.
Thus, as long as these simple human factors remain relevant, social networking sites would continue to exist (and even flourish!). Of course, it is "different strokes for different people" - and that would explain the diversity of the social networking sites that we see out there now.