Privacy International has the details:
Despite Facebook claim, “Download Your Information” doesn’t provide users with a list of all advertisers who uploaded a list with their personal data.
As a user this means you can’t exercise your rights under GDPR because you don’t know which companies have uploaded data to Facebook.
Information provided about the advertisers is also very limited (just a name and no contact details), preventing users from effectively exercising their rights.
Recently announced Off-Facebook feature comes with similar issues, giving little insight into how advertisers collect your personal data and how to prevent such data collection.
When I teach cybersecurity tech and policy at the Harvard Kennedy School, one of the assignments is to download your Facebook and Google data and look at it. Many are surprised at what the companies know about them.
Read more: schneier.com