On October 8, Twitter informed users via a blog post that their phone numbers and email addresses may have been used for targeted advertising. The email addresses and phone numbers used were supposed to be for security purposes. What you need to know
Twitter may have used your email address and phone number that were provided for security purposes for targeted advertising. As of September 17, the issue has been resolved and data provided for security purposes will no longer be used for advertising. Instead, it was used by Twitter’s Tailored Audiences and Partner Audiences advertising… Twitter apologized for the mistake and ensures users that their data was never shared externally. While that’s bad enough, it’s even worse that the email addresses and phone numbers were provided for security purposes, such as two-factor authentication. The post goes on to say that the “data may have inadvertently been used for advertising purposes” and that third-parties never had access to the data.