Help your child understand that getting ready for college is like getting ready for a job interview. In addition to clean, pressed clothing and an impressive application, you also need a clean, impressive social media image.
A clean media image has as much to do about what they post as what their friends are posting. Although they can't control what others post, they can control who can post. When they have a friend who posts expletives and unflattering images, they can do something about that.
Encourage your child to be careful about pictures they post and watch who they follow on Twitter. Keep in mind that some messages would be better suited for a private email rather than posting on a page that others can see. People who don't know your child may misinterpret what they read. Encourage your child to review settings related to what others can see.
Keep in mind that your teenager has limited life experience and has less awareness of what forever means. Help them realize that what they feel passionate about posting today could hurt them tomorrow.