The "knowledge graph" and "personal knowledge base" community is working on it! Although in a lot of little projects; nothing unified that I know of. Here's a project I worked on for a while, which lets you maintain a Neo4j database via an Emacs frontend:

https://github.com/synchrony/smsn

Here's something I'm working on now -- extremely expressive, but no app yet:

https://github.com/JeffreyBenjaminBrown/digraphs-with-text/b...