Myspace/Facebook vs. Plaxo/LinkedIn

Social networking sites have become the new way to stay connected. With everyone leading such busy lives, it has created a quick and easy way for people to be able to keep in touch. I believe that the popularity of myspace/facebook is far greater than plaxo/linkedin. I say this because I actually have heard of and used myspace/facebook before whereas I have never with plaxo/linkedin. I do think that each of these sites are great when used for what they are intended. I view myspace/facebook to be more personal rather than professional - a way for friends to stay in touch. I'd like to use plaxo, however, as a way I stay connected to the other professionals I've met during this time.