Playing around with the idea since September, the micro-blogging company has made Twitter an ad-free, or nearly ad-free, experience to a small group of some of its most prominent and active users.

Twitter selects the no-ad or low-ad group by a variety of criteria, including the volume and reach of the tweets they generate. Twitter representative Will Stickney said: "We are constantly looking at constraints and adjustments to optimise which ads are shown and how often."