Micro-blogging systems have become a prime source of information. However due to their unprecedented success, these systems have to face an exponentially increasing amount of user generated content. As a consequence finding users who publish quality content that matches precise interests is a real challenge for the average user. We present in this article a recommendation score which takes advantage of the social graph topology and of the existing contextual information to recommend users to follow on a given topic. Then we introduce a landmark-based algorithm which allows to scale. Our experiments confirm the relevance of this recommendation score against concurrent approaches as well as the scalability of the landmark-based algorithm.