Several different providers are offering universal login schema, including Microsoft, Linkedin and Facebook. Apparently, none is monetizing this phase while there are plenty of opportunities to be leveraged.
Let's analyse a simple scenario where Alice, a Spotify user, decides to login using her Facebook account. Facebook knows Alice's geo-location, Alice's profile, Alice's past postings and likes. So Facebook has a lot of information to be mined for serving ads while Spotify does not have the same information. In other words, Facebook can sell ads to Spotify's users. So this could be the "Adsense" program for Facebook.
So my question is why Facebook is not opening this ads platform?