> Ad Measurement. An advertiser can choose to add the Facebook Pixel, some computer code, to their site. This allows us to give advertisers stats about how many people are responding to their ads — even if they saw the ad on a different device — without us sharing anyone’s personal information.

So I can look at an ad in Mobile Safari, and go to the advertiser's website in Chrome on my mac, and Facebook counts that as a referral, because I'm logged into FB on both browsers. Makes sense, but sort of creepy.