"We use the information people provide to offer a better, more personalized experience on Facebook, including ads," a Facebook spokesperson said in a statement to TechCrunch. "We are clear about how we use the information we collect, including the contact information that people upload or add to their own accounts. You can manage and delete the contact information you've uploaded at any time."
Last year, Facebook began giving users more 2FA alternatives beyond a code sent to your phone, including USB key support, followed by the ability to use third-party authenticator apps in May. Users may have opted for those methods over supplying their phone number if they knew what Facebook was up to. The company also came under fire in February for spamming 2FA phone numbers with codes. It blamed that on a bug.