The rise of online social networks has massified the notion that Web users are active publishers of their own content. Users now constantly share pictures, videos and their objective and subjective perceptions of the world they live in. In particular, microblog platforms such as Twitter have made online social networks an extremely rapid environment for disseminating, almost in real-time, news and personal experiences. The democratization of content publication has brought several new challenges, among these, the analysis of large volumes of streaming data and the understanding of higher level abstractions such as sentiment, opinions and events. In this talk, I will discuss some examples of how social knowledge can be extracted from microblog content, such as, credibility prediction, event detection, and the analysis of cultural differences between countries.