Not everyone in the 21st century thinks about the negative effects of having social networking accounts - but simple things like not setting your privacy settings properly or liking someone's photo can have bad results.
Cyberbullying can be a problem as people can take advantage of the fact that there is no one who can effectively stop the bullying when it happens, due to everything being performed behind a screen. The only way for a bully to be stopped is if they are reported and victims may be too intimidated to do it.
Social networking can also ruin relationships as people may get jealous if they find out their boyfriend or girlfriend is exchanging messages with other people.
It can also be a waste of time as people can visit a site to check on thing and end up spending the whole day 'behind the screen' and as a result, not doing anything useful with their lives.