Why not for IDP discovery?

SocialHistory.js exploits a CSS leak to determine which social networks users have visited.

SocialHistory.js works by exploiting the feature in modern web browsers that automatically uses a different link color for visited links. The JavaScript loads up a bunch of links from a list of top social media sites in an iFrame and looks at which have been visited based on the change in link color. From there, it can assume which you are most likely to use