With the popularity of social network, an increasing number of users attempt to find their interested web services through service recommendation, e.g., Collaborative Filtering (i.e., CF)-based service recommendation. Generally, the traditional CF-based service recommendation approaches work, when the target user owns one or more similar neighbors or friends (Neighbor and friend are interchangeable in the rest of paper) (i.e., user-based CF), or the target user's invoked services own similar services (i.e., item-based CF). However, in certain situations, similar neighbors and similar services are absent from the user-service invocation network, which brings a great challenge for accurate service recommendation. In view of this challenge, a novel recommendation approach SBT-SR (Social Balance Theory-based Service Recommendation) is put forward in this paper. Concretely, for the target user, we first determine his/her "enemies" (antonym of "friend", i.e., the users who have opposite preference with target user), and then look for the "potential friends" of target user, based on the "enemy's enemy is friend" rule in Social Balance Theory. Afterwards, the services preferred by "potential friends" are recommended to the target user. Finally, through a case study and a set of experiments, we demonstrate the feasibility of our proposal.