Folksonomy systems and social bookmarking tools are rapidly spreading on the Web. Their steady increase reveals a more dynamic and interactive space, in which users can individually and freely share online resources and assign terms to them. In this paper, we address the problem of how to exploit these rich systems to enrich existing ontologies. Our approach taps into external tools such as the sense inventory WordNet to fix mistakes resulting from the free tagging and takes advantage from semantic relationships between tags to recognize resources' context. We carried out evaluations that mainly paid attention on the particularly challenging problem of resources' ambiguity to prove that not all resources shared in folksonomies are relevant resources for Dbpedia ontology. In this respect, it is worth of mention that our pre-processing step increases precision values to achieve an average precision of 80%.