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 28,830 websites that are Google Analytics Custom Variables Customers. We know of 7,775 live websites using Google Analytics Custom Variables and an additional 21,055 sites that used Google Analytics Custom Variables historically and 1,581 websites in the United States.