As a random example, Fiat seems to sell all sorts of information.
Here are their opt out instructions:
> * You may opt out or control how your information is used by sending an email to dprivacy@fcagroup.com.*
Since they don’t say what information they require, it seems likely this will take multiple round trips.
Apparently Facebook recommends completely deleting your account to opt out, but that presumably won’t stop them from performing ad targeting, or creating shadow profiles for non-existent users. (Shadow profiles clearly exist, since they use them during account creation to recommend new friends).
Similarly, how do I opt out of Google facial recognition that they do on behalf of other people’s accounts, nest cameras, etc?