Twitter's mobile advertising arm enables its clients to use a hidden, undeletable tracking number created by Verizon to track user behavior on smartphones and tablets.
Wired and Forbes reported earlier this week that the two largest cellphone carriers in the United States, Verizon and AT&T, are adding the tracking number to their subscribers' Internet activity, even when users opt out.
The data can be used by any site – even those with no relationship to the telecoms -- to build a dossier about a person's behavior on mobile devices – including which apps they use, what sites they visit and for how long.
MoPub, acquired by Twitter in 2013, bills itself as the "world's largest mobile ad exchange." It uses Verizon's tag to track and target cellphone users for ads, according to instructions for software developers posted on its website.
Twitter declined to comment.