One of the general motives for people to use social network sites is self presentation. However, the need of people to create multiple identities causes few practical problems when it concerns online communication. None of the existing social network sites provides the need for creating multiple identities for different social groups. Some sites like linkedin only intent to serve one social group and others, like Facebook, do not make any difference at all. What information you give about yourself depends on the target group you have. You do not want to share embarrassing holiday pictures with your professional network and inside information about your professional career can be uninteresting for your friends. Since a few months rumors about a whole new social network site developed by Google is going to solve this problem go round.
According to several scientists like Goffman (1959) and Jung (1953), people have multiple identities and more possible selves to show. Which side of our self we show, how we show it and how many we show depends on the situation we are in. This assumption, says that identity is not a fixed thing. We can influence it ourselves. With modern technologies in communication this becomes even more easy than it was when a high level of social presence still was required for self presentation.
In my opinion the selective self presentation allows us to manage our online identity in the most optimal way because it makes it easier to maintain your real life social structure in the online environment. Keeping the social structure will make communication through social networks more like in real life and therefore more social because the same level of intimacy can be maintained.