Since about September, Facebook has offered its advertisers a powerful new way to track its users as they surf the Web: It's called "phone number retargeting." The move came after Facebook made a big effort to collect its users' mobile phone numbers to prevent security breaches.
More recently, according to AdExchanger, Facebook has combined phone retargeting with a new "conversion pixel" — basically a type of tracking device — within ads displayed on Facebook.
The combination of phone retargeting and conversion pixels allows advertisers to target you directly with ads and then measure exactly how you respond to them, whether by clicking, ignoring, or buying something from the advertiser's site.
Some advertisers have been doing this kind of thing on other websites for years.
But most Facebook users don't know it's going on within Facebook. Instead, they believe the primary reason Facebook prompts them for a mobile phone number is to prevent account hacking, and to allow users to upload photos and make status updates from their phones.
In fact, earlier this year, Facebook began asking every user for a phone number for "security" purposes. Here's what Facebook says about that:
But Facebook has since made those phone numbers available to advertisers as part of its new Custom Audience targeting product. "Audiences can be defined by either user email address, Facebook UIDs, or user phone numbers," the product states.
http://finance.yahoo.com/news/remember-facebook-wanted-phone-number-150738396.html