Facebook bypass of the cache servers, Check who visits your profile! The vulnerability is a security flaw in Facebook in relation to their cache servers. This ruling, allows access to raw browser requests of our friends, allowing private information of these people (web-bug), or use as a bridge to take advantage of other external vulnerability (CSRF) Facebook will save many security issues (including privacy), gaining full control over who goes and where it goes (data mining) This is not a zero cost to Facebook.
Source: https://thehackernews.com/2011/03/facebook-csrf-security-flaw-check-who.html