No one posts unflattering pictures of themselves. Facebook profile pictures are very carefully selected. They represent an image the person wants to convey to the world, so they choose a picture taken in the best light, from the best angle, and on their best day. They most likely look a little skinnier than they are in real life.
They know how to pose. Those other girls on Facebook know the same "rules" you've memorized. Jut out your shoulder and place your hand on your hip. Turn slightly to the side. Suck in your stomach. Chin up slightly so as to prevent a double-chin.
Pictures are taken with Facebook in mind. Those group pictures that look oh-so-casual? There were multiple takes involved.
Pictures are taken with careful preparation. Maybe you feel bad when looking at these photos because you're sitting there in your pajamas and everyone looks so glamorous. They're sitting at home in their pajamas doing the same thing. Facebook pictures are likely taken in clubs after hours of preparation before going out. They don't normally look like that.
Facebook appeals to college students because let's face it, we're prone to body-snarking and being self-conscious. Use it to keep in touch with friends at other girls, and post inside-jokes on your friends' walls, but don't let it become a detriment to your self-esteem.