Earlier quoted context omitted.
> you target Facebook/Google ads specifically at that email address That is scummy as hell and might even get you in trouble when it comes to the GDPR if you're operating in the EU. If I sign up for your web service the last thing I want is Facebook/Google knowing that fact.
This is a core feature of every ad platform I've seen and is absolutely not a violation of the GPDR since users are giving consent when they signup. You've signed up for a web service and never seen ads on other sites for it ? Very strange.
So yes, technically you can ask the user for consent, but it has to be explicit ("we'd like to share your e-mail/phone number with our advertising partners such as Facebook, accept/decline?") and I can't imagine anyone in their right mind consenting to that.
> You've signed up for a web service and never seen ads on other sites for it ? Very strange.
I sign up for stuff only when I have no other choice for exactly this reason, and often provide fake details. Reminds me of an ex-client where they had an issue with their potential customers not providing the right contact details because they're afraid we're going to spam them. "But do we actually spam them? -Yes."