Oh thats grim:

By 2013, Netflix had begun entering into a series of “Facebook Extended API” agreements, including a so-called “Inbox API” agreement that allowed Netflix programmatic access to Facebook’s users' private message inboxes, in exchange for which Netflix would “provide to FB a written report every two weeks that shows daily counts of recommendation sends and recipient clicks by interface, initiation surface, and/or implementation variant (e.g., Facebook vs. non-Facebook recommendation recipients).

Netflix pays fb for ad space
Fb gives Netflix DMs to better target ads
Netflix gives fb back clickthrough and behavioral data, including data from rival advertisers (read: google)

So fb gets to say they dont use private messages for targeting ads, they just share them with whoever can give them more data on you, which is ofc fine.

https://arstechnica.com/gadgets/2024/03/netflix-ad-spend-led-to-facebook-dm-access-end-of-facebook-streaming-biz-lawsuit/