Advertising. Definitely first party data for targeting. An advertiser takes some data from its CRM, sends it to the big social sites and google, and then uses the list to target those folks specifically or create look-a-likes. Actual cross device targeting (because people are logged in), extremely personalized and relevant.
This is coupled with a move away from cookies[0].
0. https://adwords.googleblog.com/2017/01/making-youtube-better...