Users of JSON LD

• Google Knowledge Graph Search API, Actions, and many more
• BBC, see also this discussion thread
• Microsoft
• Yandex
• Digital Public Library of America platform
• Globo.com
• Jus.com.br
• Parse.ly Analytics
• Web Payments CG specs
• JSON Activity Streams 2.0
• Open Badges 1.1
• OCLC WorldCat (details)
• Enel
• Europeana
• seevl
• Schema.org
• rdf4j Sesame
• Apache Jena
• Apache Stanbol
• IKS project:
  • VIE
  • Create
  • Palsu
  • Proggis
• UKGovLD Registry
• The U.S. Government via Project Open Data Metadata Schema v1.1
• VIVO and Harvard Catalyst (details)
• Dropbox, Foursquare (use it for Actions in GMail)
• kTBS (Kernel for Trace Based System)
• Semargl
• OpenLink's
  • Virtuoso (DBpedia)
  • URIBurner
  • Vapour
• su4j
• openknowledgegraph.org
• graphitejs
• Beakit
• directory.occupy.net
• IMS Global - Learning Tools Interoperability v2.0
• Learning Registry
• Linked Data Platform
• Web Annotation Data Model
  • Open Annotation
  • Annotopia
  • Wf4Ever Research Object Bundle
  • Apache Taverna (incl namespaces)
• currency2currency
• RDF Translator
• National Library of Sweden
• SMART Platforms
• grids.by
• LevelGraph-JSONLD
• visuaLOD
• econ-ws
• Open Digital Rights Language (ODRL)
• dbpedia lite
• National Science, Technology, Engineering, and Mathematics Education Digital Library (NSDL)
• CiNii Books
• Linking Open Bibliographic Data (lobid.org)
• Fiona
• FMQL 1.1 (FileMan Query Language)
• HealthData.gov
• K-samsök (Swedish Open Cultural Heritage)
• There exist (optional) modules for:
  • Drupal
• UK Ministry of Justice Court and Tribunal Finder (planned: Tribunal Decisions)
• Profium Sense
• BBNS, Digital Heritage of Novi Sad
• Standard Analytics' RJSONLD (JSON-LD packaging tool for R)
• Learning Resource Metadata Initiative (LRMI)
• Joshfire Factory
• International Image Interoperability Framework: Image API 1.1
• International Image Interoperability Framework (IIIF): Metadata API
• Open Data @ Tor Vergata
• IOTDB
• UniProt
• LocaliData
• Language Application Grid
• Alveo Virtual Laboratory
• PeerJ
• iRail
• Encyclopedia of Life
• OpenGovLD
• Open Archives Initiative Object Reuse and Exchange (OAI-ORE)
• theThingbox.io
• Culturegraph
• parse.ly
• prefix.cc
• Blikk
• Numishare
• 2inspire
• OpenRisk (Open Risk API)
• Wistia
• Tasytt
• API Platform
• Vital (see also post on LinkedIn)
• DIG Project (USC/ISI)
• Carbon LDP
• WRIO Internet OS

And these projects are considering/discussing it:

• WikiData
• Helios
• unhosted.org
• Midgard
• gleams - RSS, OPML and JSON feed publishing and aggregation platform that emphasizes minimalistic design and architecture, loosely connected decentralized networks, and open protocols. Planned are p2p payments, multi-network hashtag handling and various interoperable components for social and private messaging.
• Profiles Research Networking Software
• Open Context: Data Publication for Cultural Heritage and Field Research