Twitter says some of its users’ phone numbers and email addresses provided for account security may have been used accidentally for ad targeting. Twitter does not know the exact number of people affected in this incident. The company apologized for this error and says that it’s taking measures to make sure that a similar mistake will not happen again. Twitter’s Tailored Audiences allows advertisers from all over the world to send targeted ads to customers they have in their marketing lists based on information such as phone numbers. Facebook used the phone numbers its users added for two-factor authentication as an additional targeting vector.
Source: https://www.bleepingcomputer.com/news/technology/twitter-apologizes-for-using-your-phone-number-for-advertising/