This is a great debate starter! http://www.dailymail.co.uk/news/article-1153583/Social-websites-harm-childrens-brains-Chilling-warning-parents-neuroscientist.html
Be sure to read the numerous (and often humorous) comments that follow.
This article presents a view shared by many teachers who are concerned with students spending too much time online. Will we have to teach face-to-face social skills?
My initial reaction to reading the article (and the reactions of many commenters) is this argument was made in the 50's when rock and roll music became popular. Surely this music would cause the end of society as we know it!
There are some legitimate concerns that will only be addressed by scientific research. In the mean time, I believe it is a parent's responsibility to make sure their children spend time away from the computer (or television or video game or...) and are socially interacting with other children and adults.