query: SELECT ?country ?countryLabel (count (DISTINCT ?_Instagram_username) as ?count) 
  WHERE {
    ?museum wdt:P31/wdt:P279* wd:Q33506 .
    ?museum p:P2003 ?statement.
    SERVICE wikibase:label { bd:serviceParam wikibase:language "en". }
    OPTIONAL { ?museum wdt:P625 ?_coordinate_location. }
    OPTIONAL { ?museum wdt:P2003 ?_Instagram_username. }
    ?museum wdt:P17 ?country
  }
  GROUP BY ?country ?countryLabel 
  ORDER BY DESC(?count)