[threeseed]

None of this makes any sense.

Facebook Marketplace is a marketplace. They don't control the products that are being advertised. All they can control is the weighting of ads they run on certain products e.g. "AirPods are popular, increase ad spend to 20% of total".

But they could do this by simply optimising for ROAS just like every other company does.

If Facebook is sharing AcmeShop data with Marketplace, and it's a very big if, it would still be of almost no value.

[dgroshev]

They don't control the products that are being advertised, but they make personalised ads for Marketplace products more effective by using data they collect elsewhere. If the ads are more effective, Marketplace is better at shifting goods, undermining the competition because their competition is forced to share data with Facebook through Facebook's (not Marketplace's) market position.