Here is an interesting article on the rise of the Web hermit, which is a person who may be socially isolated because he or she makes Web activities the main focus of their life, rather than the real world. As a simple example, you can now do all of your banking and bill paying online, so that you never have to go to a bank and interact with a human bank teller. Or instead of playing cards with live friends on Friday night, you play cards using an avatar in an online game in a 3D simulated environment.
There is a growing set of data that suggests that we are becoming more socially isolated, but for a whole variety of reasons. The Internet is not always the direct cause, but in some cases, it may be the indirect effect of Internet services. I tend not to worry too much about these things. We are, at our core, social beings, and over time, as the "newbie" effect of these things wears off, the pendulum usually swings back the other way. As we can do more and more routine stuff online, we may come to value "real" social activities even more and begin to make more effort to stay connected to the real world.