Content-centric social Web sites, such as discussion forums and blog sites, have flourished during the past several years. These sites often contain overwhelming amounts of information that are also being updated rapidly. To help users locate their interests at such sites (e.g., interesting blogs to read or discussion forums to join), researchers have developed a number of recommendation technologies. However, it is difficult to make effective recommendations for new users (a.k.a. the cold start problem) due to a lack of user information (e.g., preferences and interests). Furthermore, the complexity of recommendation algorithms often prevents users from comprehending let alone trusting the recommended results. To tackle these above two challenges, we are building a social map-based recommender system called Pharos. A social map summarizes users' content-related social behavior over time (e.g., reading, writing, and commenting behavior during the past week) as a set of latent communities. For a given time interval, each community is characterized by the theme of the content being discussed and the key people involved. By discovering, ranking, and displaying the most popular latent communities at different time intervals, Pharos creates a time-sensitive, visual social map of a Web site. This enables new users to obtain a quick overview of the site, alleviating the cold start problem. Furthermore, we use the social map as a context to help explain Pharos-recommended content and people. Users can also interactively explore the social map to locate the content in which they are interested or people that are not being explicitly recommended, compensating for the imperfections in the recommendation algorithms. We have developed several Pharos applications, one of which is deployed within our company. Our preliminary evaluation of the deployed application shows the usefulness of Pharos. © 2011 ACM.