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Not just Facebook. As an ex-Hubspot employee this was a feature people cared about (email list uploading/processing). HS would generally try to scan the list and make sure it wasn't a huge pile of spam recipients but I'm not sure how many others in the CRM space take that extra step.

I believe this is almost table stakes for an ad platform these days. Marketers expect ever-more precise targeting to get better CPCs/CPAs/ROAS. This is especially true as an ad platform becomes more popular and CPMs naturally rise due to increased competition in the auction. When a company like FB/GOOG innovates in the ad targeting/measurement space every company will follow suit because their customers (advertisers) are going to ask for it. I believe market forces will continue to drive innovation in ads targeting until someone steps in and draws a line in the sand (eg regulation via the governement).

Why not?

Regular users used to be able to search for people using email addresses, not anymore.