Facebook has finally implemented a tool that I have tried very hard for them to implement. This is a transparency tool on the tracking they do through Pixel (on website) and SDK.
It is mandatory for them to tell such information if asked, under European law. When I asked in Jan 2016 (through the little-used Safe Harbor) they told me it was essentially too difficult to implement.
Four years later it is there, but it took a lot more efforts. Among other steps it required a Senator asking in follow up questions to Zuckerberg to resolve discrepancies between his testimony and my own at the UK Parliament! In fact it was so hard that there is even a wiki entry to describe the timeline of events surrounding that tool.
In any case, the link https://www.facebook.com/off_facebook_activity/ then click on the icons should directly get you to a list of all the different third parties Facebook has communicated information to.
Now the challenge is to make this list useful for advocacy. This can be done!!! I have a ton of ideas, really, starting with pairwise comparison between users. First step: build a very basic scraper to pull this list from the site.
If you can help, sign up to the forum and introduce yourself below!