to me there are more bad than good. children's minds become slower and less active than a child who is being a child just as a normal child is expected to be.(run, play games....basically to use their imagination) and not to occupy their minds with something that would be of no benefit at all to them.
it also damages the eyes. well Face book can cause allot of problems in ones life. yet it also can help you keep in contact with far relatives.
when children tend to be into these things from a young age they also become or pretend to be mature form a young age..and then when they are grown and expected to be mature then they want to now start enjoying their youth when its almost over and too late....