One of the objectives of the Language Technologies for Life-Long Learning (LTfLL) project, is to develop a knowledge sharing system that connects learners to resources and learners to other learners. To this end, we complement the formal knowledge represented by domain ontologies with the informal knowledge emerging from tagging. We have developed an ontology enrichment pipeline that can automatically enrich a domain ontology using data extracted from social media applications, similarity measures, DBpedia knowledge base and several heuristics. An evaluation of the resulting ontology has been carried out.