How to find out how much customers uses wishlists?

SELECT
	(SELECT COUNT(DISTINCT w.wishlist_id)
	FROM wishlist w
	INNER JOIN wishlist_item i ON w.wishlist_id = i.wishlist_id)
/
	(SELECT COUNT(*) FROM customer_entity)
* 100 as '%'

See also:

• How to find out the most active wishlist users in your store?
• How to find out how much customers uses the product comparison feature?