There are probably studies out there by universities and other organizations about how teens insecure about X, Y or Z tend to enjoy consuming products A, B or C. Facebook can easily recoup this information with pages and posts teens tend to like. They could also analyze the tones of their comments with some AI though this is not as advanced as we may think.

There are probably a gazillion other variables they can play with to end up with pretty good results about what teens are insecure about and how they tend to behave on Facebook and specially what kind of ads they're most to likely to click on. Note that they don't sell this info to their clients, they just show the ads to the right people.