Since their introduction, Social Networking Sites (SNS) have fundamentally changed the way people communicate and interact with each other. Alongside the wealth of applications they offer, SNS also bring new dangers due to the nature of digital information and its personal character. In particular, personal data can be copied, modified and disclosed without their owner's consent or without the explicit acknowledgment of other possible co-owners. In this paper, we investigate tools that can help users to maintain the sovereignty of their data on the World Wide Web. We also introduce PrivacyMarker, an approach drawing on the concept of provenance and accountability to protect user privacy on SNS.