Even if people do not reveal their personal information online, others may do so. In this paper, we consider the problem of involuntary information leakage in social network services and demonstrate its seriousness with a case study of Wretch, the biggest social network site in Taiwan. Wretch allows users to annotate their friends' profiles with a one-line description, from which a friend's private information, such as real name, age, and school attendance records, may be inferred without the information owner's knowledge. Our analysis results show that users' efforts to protect their privacy cannot prevent their personal information from being revealed online. In 592,548 effective profiles that we collected, the first name of 72% of the accounts and the full name of 30% of the accounts could be easily inferred by using a number of heuristics. The age of 15% of the account holders and at least one school attended by 42% of the holders could also be inferred. We discuss several potential means of mitigating the identified involuntary information leakage problem.