Personal mobile devices keep private information which people other than the owner may try to access. Thus far, it has been unclear how common it is for people to snoop on one another's devices. Through an anonymity-preserving survey experiment, we quantify the pervasiveness of snooping attacks, defined as "looking through someone else's phone without their permission." We estimated the 1-year prevalence to be 31% in an online participant pool. Weighted to the U.S. population, the data indicates that 1 in 5 adults snooped on at least one other person's phone, just in the year before the survey was conducted. We found snooping attacks to be especially prevalent among young people, and among those who are themselves smartphone users. In a follow-up study, we found that, among smartphone users, depth of adoption, like age, also predicts the probability of engaging in snooping attacks. In particular, the more people use their devices for personal purposes, the more likely they are to snoop on others, possibly because they become aware of the sensitive information that is kept, and how to access it. These findings suggest that, all else remaining equal, the prevalence of snooping attacks may grow, as more people adopt smartphones, and motivate further effort into improving defenses.