Google Analytics Custom Variables Usage Statistics

Custom variables are name-value pair tags that you can insert in your tracking code in order to refine Google Analytics tracking.

https://developers.google.com/analytics/devguides/collection/gajs/gaTrackingCustomVariables?hl=en

Analytics and Tracking

Google Analytics Custom Variables Customers

Get access to data on 31,321 websites that are Google Analytics Custom Variables Customers. We know of 12,303 live websites using Google Analytics Custom Variables and an additional 19,018 sites that used Google Analytics Custom Variables historically and 3,346 websites in the United States.