There are both positive and negative aspects in the use of social media in news and information dissemination. To deal with the negative aspects, such as the spread of rumours and fake news, the flow of information should implicitly be filtered and marked to specific criteria such as credibility, trustworthiness, reputation, popularity, influence, and authenticity. This paper proposes an approach that can enhance trustworthiness and content validity in the presence of information overload. We introduce Alethiometer, a framework for assessing truthfulness in social media that can be used by professional and general news users alike. We present different measures that delve into the detailed analysis of the content, the contributors of the content and the underlying context. We further propose an approach for deriving a single metric that considers, in a unified manner, the quality of a contributor and of the content provided by that contributor. Finally, we present some preliminary statistical results from the examination of a set of 10 million twitter users, that provide useful insights on the characteristics of social media data.