Social bookmarking sites allow users to store and tag bookmarks in order to access them later. Also, tagged bookmarks designated as public by their contributors are available to all users. In this paper, we explore the relationships between user motivations and contribution characteristics to understand why users contribute to the public repository of tagged bookmarks when it is not mandatory to do so. We find that users' self-oriented motives are associated with the quantity and quality of contributions for self, but other-oriented motives are associated only with the quality of contributions for others. In other words, users contribute tagged resources for other users only if they believe they will be useful for those users. Moreover, higher quality contributions for others do not diminish the quantity of such contributions. We also find that there is a spill-over effect from quality of contributions for self to quality of contributions for others.