For visitors, Dave stated that the biggest issue is privacy. To me, this makes sense, and these fears are increasingly justified now with the amount of violation people are facing, including identity theft, cat-fishing, etc. However, the benefits to being an online resident seem to out weight the potential consequences. Having a good online presence is a great way to network, sell yourself, and make yourself known. This seems to be the appeal to residents. They enjoy the community and the capabilities it provides them.
As for me, I believe that I am making the transition from being a visitor to a resident. In this day, it is increasingly important to make a good name for yourself online. I want potential employers will want to be able to "google" me and find nothing but amazing things. As for my students, I believe that it is their choice. While I fully intend to teach them about the great resources and tools the internet can provide, I want them to be able to make a decision about it that they feel comfortable with. I may not be completely sold or comfortable with the idea of having to maintain multiple social networks, I am taking it one account at a time.