FACEBOOK (Reporting Sexting or Bullying)
It’s against Facebook’s rules to post a naked picture of someone under 18. If someone’s shared a naked picture of you on Facebook, you can report it and they’ll try to get it taken down.
Fill out Facebook’s form to report an image or select what best describes what you want to report. Facebook won’t tell the person you’re reporting that it was you. They’ll tell you in your Support Inbox if they’ve been able to take the picture down.
On Facebook messenger, log in to your account and fill in the form if you want to report images being shared.
BLOCK AND REPORT BULLYING
You can remove tags or block people who are sending you nasty messages. You can also report abusive posts or groups so Facebook can take them down. Reports are anonymous so the person doing the bullying won’t know who reported it. Facebook will tell you in your Support Inbox if they've been able to help.
Have a look at Facebook's page on privacy to change your privacy settings and control who sees your profile and posts.
You can also hide someone on Facebook. This means that you won't see their posts in your newsfeed, but you'll still be friends with them.
If you don’t have a Facebook account, you can still make a report to Facebook.