I find on-line social networking sites really exciting. So what I'm going to say will come as a surprise. The long term implications of these sites that I can think of are mostly negative ones.
The Internet has served to liberate some, myself included. But it has also fuelled a widening income, generational and experience gap. With the global recession, the days when a smart phone with a mobile Internet connection was affordable for all, have disappeared. This has created a rift in society. Peer pressure doesn't always vanish so quickly.
When some people (at least in my country) are currently struggling to afford transport to attend their work place - the attractions of fancy gadgets (the hardware of social networking sites like your Blackberry and your ipod) may soon be on the wane. Will social networking sites fall out of fashion too? It is quite feasible that some of the companies involved might fall victim to the global credit crunch.
If one section of society is organising meetings, social events and exchanging information via text and internet messaging and social networking sites and the other group in society can't or won't get to grips with these technologies then greater social isolation will be the end result. In the culture of my country it is accepted that certain social venues are suitable for certain age groups. Night clubs for the under twenty fives. Bowling greens for the over seventies. Other cultures are different and I'm pleased to say that people of different age groups tend to mix a lot more.
Every single winter there is at least one report in the papers about an elderly person who died in their home, unnoticed, due to cold, or old age. Often these people live alone and their death remains undiscovered for days or even weeks. Maybe neighbours or family didn't notice, or even care. No amount of 'facebooking' will solve that problem.
On a positive note, with the challenges of climate change, it is quite possible that sensible use of online social networking sites would be one factor that would help us all to reduce our carbon emissions further. It's not always necessary to physically be in a place to have a meeting, and it would save air miles at least.
For people who of necessity spend a great deal of time at home, for example if they have caring responsilbities or a disability of some sort, social networking sites can be a lifeline. I can also imagine that their role in schools and in the education system could be expanded to benefit projects such as distance learning.