Is this any surprise? Just a few hours ago the Acton article on the front page[0] talked about them doing this in WhatsApp:
> Later he learned that elsewhere in Facebook, there were “plans and technologies to blend data.” Specifically, Facebook could use the 128-bit string of numbers assigned to each phone as a kind of bridge between accounts. The other method was phone-number matching, or pinpointing Facebook accounts with phone numbers and matching them to WhatsApp accounts with the same phone number.
> Within 18 months, a new WhatsApp terms of service linked the accounts and made Acton look like a liar.
Companies like this, and Facebook in particular, are desperate to connect identities. Phone numbers are an incredibly useful way to do so. Most people only have a couple of them, their re-use rate is slow, they get entered into forms all over the place, and they're usually valid (because they were provided as a primary method of contact).
In this case advertisers have an identity (and phone number), Facebook wants to match on that value. They're going to do it any way they can.
It may not be ethical, but the carrot is right there and it's naive to think you can give them your identifying number and they're going to turn a blind eye to it.
[0] https://news.ycombinator.com/item?id=18074690