New ties are often formed between people who already have friends in common. Though the social sciences have addressed the effects of existing structural patterns on the formation of new ties, less attention has been given to ties in directed networks. Drawing from the microblogging service Twitter, we conducted a web-based experiment in which subjects were asked to rate their interest in forming ties to other people, blind to existing network connections between them. We show that two structural characteristics, transitivity and mutuality, are significant predictors of the desire to form new ties. Our findings shed light on tie formation, especially in online networks.