Alex mentioned that may be some confusion between foaf:Person and sioc:User - I hope that this picture showing the alignments between SIOC, FOAF and SKOS will help to clarify that a foaf:Person can own many sioc:User profiles (via the foaf:holdsOnlineAccount relationship). I have also included some connections from SIOC to the SKOS ontology (aliman, hope you don’t mind the logo!).