This presentation introduces the notion of semantic engagement. It illustrates four different ways semantics can be used for effective and efficient on-line communication. First, we discuss semantic analysis of communication items based on 'classical' semantic, such as natural language processing. Second, we look at semantics as a channel, viewing Linked Open Data vocabularies not only as terminological assets but as communication channels. Third, semantics provide the methodologies and tools for content modeling by means of ontologies. Finally, semantics through semantic matchmaking enable semi-automatic assignment and distribution of content to channels and vice-versa.