Wikidata are fantastic. It grows like wildfire. Sadly I can not find a clue as to how relevant it is.
One traditional clue is by comparing the growth in page views. This is how often a page is requested for viewing. I am probably one of the few who enjoys browsing the information that is provided and add the statements that take my fancy.
I am convinced that Wikidata needs a large community of people making a statement. Traditionally growing page views statistics have been really stimulating. With all the available technology it should not be that hard to provide statistics for Wikidata as well.
When page view statistics are available, the success of any and all measures intended to make Wikidata popular can be measured. It is quite likely that other statistics will prove to be just as relevant in finding out what needs doing to make Wikidata popular and relevant. But that is another story.