But you sharing the data with Facebook. If I'm not on Facebook (which I'm not) you are telling them that there most likely exist a user with this email address

But you sharing the data with Facebook. If I'm not on Facebook (which I'm not) you are telling them that there most likely exist a user with this email address and an interest in your service. If many companies do this FB might even be able to build a profile of me without me doing anything This is (or at least should be) not Bueno under GDPR / data minimalization.