Who uses Wikibase? Any organization can use it to manage and open up their structured data, and many do—in particular, data-minded GLAM organizations across the world. Here's a sampling of them and their projects using Wikibase.
Libraries frequently find Wikibase well suited to their requirements: it provides librarians with powerful collaboration tools with no need for prior in-depth knowledge of linked open data principles.
Wikibase offers the greenfield conditions often needed by collections with large bodies of custom data in order to create an accurate representation of the unique information in their hands.
We see universities and research groups using Wikibase for its flexibility and ease in collaboration, and we see it helping them bring fascinating bodies of information to light.
Perhaps no subset of Wikibase's usership leaps as readily to one's mind as the scientific community: huge amounts of raw data, multiple collaborators, every project's data structure wildly different and ripe for federation.