"I can't believe you don't get it. The rules are different for the Serena van der Woodsens of the world! People expect you to party and be wild, sleep with whoever you want, run away, come back..." The price you pay for being in free-fall is everybody seeing you're in free-fall, but there are benefits. It feels real, it feels good even when it hurts, it feels like a certain kind of grace when things look like they really are: when the burlesque stops and you're just a girl, instead of a towering paradigm of willpower and chastity and perfection. Serena gets to do whatever she wants, and Blair hates that; Blair gets to coast on appearances because all her very scary shit takes place behind closed doors, and Serena hates that. You're always one or the other, because reputation is everything, so you have to make a choice: live up to the reputation, constantly getting scared and damaging yourself in that pursuit, or drag the rep down to a manageable place, and end up constantly apologizing for yourself.
Any system has rules, whether it's gender roles or heterosexuality or rich-dad/poor-dad class structure, and your option is to learn those rules and perform them properly, in order to be rewarded by that system. Get good grades and get into a good school and finish at the top of your class, and you'll be rewarded with money, for example. But any system, social or otherwise, still operates in terms of game theory. Step out of the game altogether, reclaim your space, and you forfeit the right to those rewards, because you're no longer in the running. So the deal that you're actually making, when you agree to quit the team or come out of the closet or admit aloud or publicly that you're a sexual being -- a whore, by the rules of the game -- is what shame was invented to contain and administrate. Shame exists in society for one reason only: to keep everybody playing the game.
But the reward and the price of being in free-fall, as Serena can tell you, is that the whores of the world don't really care about any of this, so they get to make their own rules, because they're playing a different game altogether. By any recognizable, mature measure, Serena is a great person: compassionate, kind, understanding, and fiercely loving. But we're not in a recognizable or mature world; we're in high school, and the rules are different: "Wait. Are we really going back there?" Always. There are things you never get to stop apologizing for. But that's not B's point: "You shot your reputation to hell a long time ago. It doesn't matter what you do, but I'm a Waldorf." Waldorfs play the game: they run to France when it gets scary, they hide their eating disorders with the help of their servants, they smile and grin with their down-low husbands, and call it love. How could Serena ever explain what it's like, out here with the rest of us? She can't, and Blair's finally found just the right place to twist the knife: reminding her that Serena will never, ever be able to buy her way back into the game. "Well, since you and your reputation obviously don't need me and my low-rent taste, you and the Waldorf name can weather the storm alone." In the silence, Blair reviews everything that just happened, the awful abyss that Serena's free-fall means, the terrible storm that's still coming. "...With pleasure," she says, but she doesn't mean it. There's no pleasure here.