Twitter has acknowledged that user phone numbers and email addresses gathered for security purposes, as part of its two-factor authentication policy, may have been used to sell ads. The company calls the move an accident. The revelation is being widely criticized for its obvious breach of user privacy, particularly since it occurred via a scenario meant to bolster user security, not violate it. Twitter did not disclose exactly when they discovered what was happening but said that as of Sept. 17, the issue has been addressed.
Source: https://threatpost.com/twitter-uses-phone-numbers-emails-to-sell-ads/149014/