the negative consequences Facebook has caused to Facebook Teen users such as how it has damaged their:
health: by going to sleep late, due to leaving hw assignments untill last minute casuing stress etc)
social skills: interaction with other is less, no family time etc
academic performance: less focus in school because they check phone every 10 minutes, less time to do HW because of checking updates on fb, looking through uploaded pictures etc.

another point that can be mentioned when talking about young adults could be the harm Facebook can cause when trying to find a job and interviewer decides to search applicant through fb to see lifesyle he or she has.