Gender inequality starts before birth. Parents tend to prefer boys over girls, which is manifested in reproductive behavior, marital life, and parents' pastimes and investments in their children. While social media and sharing information about children (so-called "sharenting") have become an integral part of parenthood, it is not well-known if and how gender preference shapes online behavior of users. In this paper, we investigate public mentions of daughters and sons on social media. We use data from a popular social networking site on public posts from 635,665 users. We find that both men and women mention sons more often than daughters in their posts. We also find that posts featuring sons get more "likes" on average. Our results indicate that girls are underrepresented in parents' digital narratives about their children. This gender imbalance may send a message that girls are less important than boys, or that they deserve less attention, thus reinforcing gender inequality.