PREFIX rdf:  <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX foaf: <http://xmlns.com/foaf/0.1/>

SELECT DISTINCT ?predicate
WHERE {
  {
    ?person rdf:type foaf:Person .
    ?subject ?predicate ?person
  } UNION {
    ?person rdf:type foaf:Person .
    ?person ?predicate ?object
  }
}