When I was in college, my room mate at the time introduced me to the world of facebook. When I first signed up I added all the people whom I knew around campus and such. Of course through time, I was introduced to the fact that people are very open about the sinful activities that produce no heavenly fruit.....even among those who called themselves Christian at the time. I was very saddened to see family members engaging in the same sinful activities as they connected me to their facebook page. Although I have never spent considerable amount of time on facebook, I have posted links to good Christian websites hoping to win the lost friends and family. Unfortunately, it seems that the heavenly kingdom does not work that way, as face to face conversation wins the lost much more effectively, especially if they are "name only" Christians. I have since deactivated my account as it seems to be more of a burden than a blessing and it is a site that promotes worship of "self" rather than of God.
Here is a good article about the deceptions involved with internet social networking.
If you are a part of a social network, I don't really consider it a "sin," however, it is wise to consider what kind of fruit the network produces. Blogging can also be a form of social network, however, there is more space to share only Christian centered writing, rather than worldly nonsense.