To promote a merchant's products on a social networking system, a promotion engine receives data from a merchant site regarding the merchant site's users' activities. The users' purchases of a promoted product are correlated with the user's activities performed in connection with a related product. An automated process running on a computer system then determines promotion criteria for the promoted product, based at least in part on the activities performed in connection with the related product. A promotion for the promoted product is generated and communicated to a social networking system, which displays the promotion to one or more users of the social networking system based on the determined promotion criteria.