Edited to reflect User to UserAccount change.

Alex mentioned that may be some confusion between foaf:Person and sioc:UserAccount - 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:UserAccount profiles (via the foaf:account relationship). I have also included some connections from SIOC to the SKOS ontology (aliman, hope you don't mind the logo!).