While it's original purpose is maintain a healthy social network between friends and family, modern day uses for Facebook have perverted the sites original intentions. As with any online activity for underage users, security measures must be taken. Younger users who are subjected to perverse materials online, may be subject to worse decisions in life.
I don't believe any social media, especially facebook is bad for you. I believe the excessive use of any social media such as Facebook can be harmful because at a young age you need to experience different avenues of learning in order to become a smarter individual. If you are constantly glued to facebook you aren't going to learn very much. You might become an expert on the lives of others but the quality of you life isn't going to be great.