Despite the popularity of social network sites based on common interests, the association between these shared interests and relational development is not well understood. This manuscript reports results of an empirical investigation of interpersonal relationships on Last.fm, a music-based social network site with a multinational user base. In addition to baseline descriptors of relational behavior, the chief goals of this study were to examine the degree to which Last.fm relationships are characterized by homophily (and particularly by shared musical taste), the extent to which communication via Last.fm is associated with other forms of communication (both offline and online), how such communication behavior is associated with demographic and relational characteristics, and whether these variables predict strength of relational development. Results indicate that although Last.fm relational partners exhibit shared musical taste, this shared taste is not associated with relational development. Rather, following media multiplexity theory, relational development is strongly and uniquely associated with communication behavior across almost all forms of communication (including Last.fm). These results suggest that shared interests may foster the creation of weak ties, but conversion of these connections to strong ties is relatively rare.