A traditional society was based on the idea that men produce and women procreate. This kept the society running because men couldn't procreate and women couldn't produce as most of the work in earlier times was strenuous, manual labour.
Women couldn't work so they needed a man to earn money for themselves. But to get a man, they needed to give something in return - sex. By collectively withholding sex when single, women could create a shortage of it for the men. So men who wanted sex needed to get a job and get married. As supply of sex diminished while demand stayed the same, its price got higher, meaning that a woman could get a better-off man in return for sex in marriage.
Purity Balls are a means to enhance this system by applying peer pressure and indoctrination from a very young age. If a 5-year-old is told something by its parents, it will more willingly accept it as the truth than a 25-year-old who might apply critical thinking and question whether this is a good idea. Moreover, after attending the Purity Ball ceremony, a girl having sex is not just disobeying her parents' will, but also breaking a promise she made – even though it was not her decision to attend the Purity Ball, but her parents'.
In today's society, the system of women procreating and men producing is outdated. There is much intellectual work that women can do, and there are more than enough people in the world anyway.
So on one hand, it's good that the facebook campaign publicly attacks Purity Balls so that people, especially women, would start opposing it more. On the other hand, in typical feminist fashion, the campaign is not being completely truthful. It's not a system created by men to oppress women, but a system that limited both sexes. By reciting this narrative that men are bad, women are good, the feminists secure themselves the support of half of the population, which is effective politics, but they bend the truth somewhat.