Maybe also don't leak your customers email addresses to FB at all.
Here is a contrived example that doesn't reflect reality:
There are 10 banks in a market. The banks have a range of new online services. Adoption of as many of these services as possible is the key to success. A viable strategy is to retarget customers (segmented by income/family status and other things that make certain services relevant) and tell them about various services. This increases the adoption of value add services substantially.
If one bank takes a moral stance, they either need to live with lower adoption of their services or spend more via less effective advertising channels.
If Facebook advertising is good enough, that one bank would go out of business, have its management replaced by shareholders or get bought out by one of the others.
This probably doesn't apply to banks, but there are businesses where effective online advertising is the difference between success and failure. All the tourist/hotel/flight booking sites, for example.
Except, of course (and this is probably the ugly truth) - as long as Facebook doesn't do creepy/embarrassing things or otherwise actively annoy their customers, the vast, vast majority of customers just doesn't care. And then there isn't really a problem.
1. Respect privacy.
2. Carefully mislead the customer to believe the company respects user privacy in a way their fuzzy morals tell them isn't a "lie," bury the truth in the fine print. Lose the handful of customers both technically savvy, principled and capable enough to not use the product while profiting from abusing the privacy of their users.
This is true in some cases, false in others. If you have 8 years worth of emails, that's what you have. If you want to segment your data based on info in your DB (people with & without a mortgage), you will probably need to use emails.
In any case, you are still leaking data. FB now know that your customer is John Lennon.
So what if they are hashed? Facebook has the solution to all the hashes.
Setting it up was a bit of a fiddle because I suck at exim, but it's no ongoing hassle.
So cpach@gmail.com and cpach+hackernews@gmail.com would both end up in the same inbox, then you can use gmail filters to separate them
Perhaps someone here has some statistics to share? It would be interesting to know how common this trick is :)
[Edit: Better wording]
Postfix allows you to choose any character as a separator.
I agree that the privacy violation is bad as it is, but I don't see how this is a good example for why it's bad.
"Facebook’s custom audience feature enables you to create an audience using your data such as email addresses and phone numbers. When using Facebook’s custom audience feature, your data is locally hashed on your system before you upload and pass such data to Facebook to be used to create your custom audience (the “Hashed Data”)."