This pin's insidious subtext: girls should not do anything that takes away from pretty-being. That girl in the back has either heard her favorite song come on, or spotted an old friend across the bar, or is singing or dancing or otherwise having a great time. This one momentary snapshot does not invalidate her implied great time at this party, nor do the smiles of the other two girls mean they're 'better' at drinking.
My cousin made reference to this, and thought it was so damn funny. You know what this sweater is made of? (These Ryan Gosling things make me laugh. Just the whole fact that it turned into a giant meme is amusing to me. And the people who write some of them are just so spot ON on the target audience for the humor. Hey Girl, FTW!)