The advent of Web 2.0, also called the Social Web, has changed the way people interact with the Web. Assisted by the technologies associated with this new trend, users now play a much more active role as content providers. This Web paradigm shift has also changed how companies operate and interact with their employees, partners and customers. The challenge for companies and research institutions is now to develop (semi-) automated tools for gathering usable and explicit knowledge from such content. With the aim of facilitating the achievement of such a challenge, in this work a platform architecture for informal learning, which is based on semantic technologies, is proposed. Such platform permits to perform expertise mining from Social Web-generated content. Given a topic of interest, the system carries out semantically enhanced operations on blog and microblog posts to identify experts in that specific topic area. The comprehensive evaluation of the tool has demonstrated very promising results and is also presented in this article. (Contains 5 figures and 1 table.)