Microsoft Cosmos DB provides a "graph interface" to a document db.
AWS claims to be a native graph DB.
It would be interesting to see performance comparisons with Neo4j and similar native graph DBs.
Amazon Neptune Generally Available | AWS News Blog
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AWS re:Invent 2017 - Amazon Neptune: Fast, Reliable Graph Database Built for the Cloud - YouTube
Amazon Neptune vs. Neo4j Comparison
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Amazon Neptune, a New Cloud Graph Database, Is Now Generally Available @ InfoQ