[cm2012]

I would say there are huge privacy differences between selling data and selling the right to advertise to their private data. The latter is quite a bit more privacy secure.

(Note, the Cambridge Analytica stuff happened in 2013 when Facebook was much more open with APIs using their data, since popular demand at the time was for FB to be less of a walled garden)

[Retric]

Even just targeted advertising is enough. Setup a campaign landing page for each campaign and anyone that clicks the link just exposed their gender or any other criteria you set up when defining the campaign.