Significant gaps exist in our knowledge of real world social network structures, which in turn limit our understanding of how to design social software. One important reason for this has been that researchers have not been able to systematically probe individuals in sufficient detail about 'who' and 'how' they interact with in the social networks they wish to study. To address this shortcoming we designed TellUsWho, a web-based social network survey tool. We explored the tool's utility by studying the social ties of 141 students. TellUsWho supported the collection of rich social network data in a relatively short time period. Within an average of 34 minutes, respondents were able to describe their egocentric ties with people they regularly keep in contact. On average, respondents listed 42 alters, for each of which they answered 27 questions, resulting in 1134 responses. This compares favorably to traditional methods, which could require up to 15 hours per subject.