Over at John Fea's blog, "The Way of Improvement Leads Home," he ponders the academic relevance of LinkedIn. I agree. Although I have a LinkedIn account, I rarely visit it, and have not updated my credentials in some time.
A better alternative for academic networking is Academia.edu, which I have found to be a great way to connect with other scholars in my field. You can upload an image of yourself and post your cv, articles, books, and other papers for scholars to view. Furthermore, you receive updates on people who begin to "follow" you, and the scholarly works that others have looked at on your webpage.
I don't have a Facebook account and do not use Twitter in order to leave more time for my research, writing, and teaching, but I have found Academia.edu to be very helpful for my research and a great way to keep in touch with other scholars.