Isolation: One can suffer from isolation as one will tend to be glued to the computer and move out of the house and meet people. Normal socializing that is interacting with people is also essential. A shy child might prefer to communicate only through Facebook only and otherwise
Putting on weight: One may put on weight also as one will sit in front of the computer for a long time and eat. Naturally, one will not have many exercises and they will suffer from obesity
Abnormal symptoms: Surfing Facebook does show abnormal symptoms among many teenagers such as depression, excessive aggression, abnormal anxiety or even normal anxiety. Those suffering from social anxiety tend to immerse themselves in front of Facebook. Teenagers usual have emotional problems and are very conscious of their self-image, their bodies, and their looks. If socially, they feel inferior they tend to avoid social interactions. Obviously, they would like to spend hours sitting writing for the Facebook
Bad for the eyes: Too much exposure to the computer is bad for the eyes. Teenagers must go out and spend time in outdoor activities and relish the fresh air, rather than remain glued to the computer
Communication with strangers: On Facebook communication, it is possible that one might end up building up relationships with strangers, who can turn out to be criminals as well. Teenagers are vulnerable and are easily susceptible to outside influences. Their lifestyles are changing and want to build up relationships with people who are very hi-fi in their outlook. In this process, they try to communicate with strangers who perhaps can be very harmful to them
Lacks emotional connection: There is lack of emotional connectivity since one does not meet the other person. Feelings, when shared in person, have more impact than on Facebook. The feelings of care and warmth are there when the communication is face to face. There is a decrease in face-to-face communication. Face-to-face communication helps better messaging
Promotes laziness: Sitting in front of the computer and browsing through Facebook can lead to laziness. It is a sheer waste of time
Causes distractions: Browsing Facebook does distract one’s mind from many other activities. One can be involved in constructive activities that keeps one’s mind healthy. Merely spending hours on Facebook and conversing via it is not healthy
Lack of body language communication: Body language conveys a lot. In Facebook browsing, there is nobody language communication as one does not interact with another person
Unhealthy sex approach: On Facebook one tends to communicate on unhealthy sexual practices. There is no healthy sexual interaction as one is not meeting each other physically. Merely communicating via Facebook can prove to be detrimental to one’s overall self-image and also to mental growth. Perverted practices can be discussed or the person on the other side may be a pervert and wants to communicate sexually. Later on he or she may take undue advantage of the person’s weaknesses and try to meet the person
http://www.paggu.com/getting-into-roots/top-10-negative-effects-of-facebook-on-teens/