How many {describes a project that uses} are to/by {Twitter} ?
What number of projects use Twitter?
How many describe a project that uses Twitter?
SELECT ( COUNT ( ?d ) AS ?w ) { ?d property:describes_a_project_that_uses resource:Twitter }
SELECT (COUNT(?sub) AS ?subs ) {
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> ?sub .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P4510> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> http://wikidata.dbpedia.org/resource/Q918 .
} 

SELECT (COUNT(?sub) AS ?value ) { ?sub wdt:P4510 wd:Q918 }
5