My number is on my profile, so that my friends can contact me shall they lose my number. Less important now that messenger exists and we can access it any time, but I put it there before it existed. It's come in handy in the past.

I also don't consider my phone number "sensitive" information I want to keep secret - it's already quasi-public and something I give out when I want people to be able to contact me.

I grew up looking people up in the physical phone book when I lost their number, fwiw.

Many companies present you with the opportunity to "Protect Your Account" with SMS, and for that protection they 'just' need your phone number.

Turns out the phone number is the best unique identifier and is the perfect key for joining up lots of disparate sources of data. It's the kind of thing that you could either sell directly or use as an index to determine things like your estimated income.

It wouldn't surprise me if Facebook has had multiple technical methods for devising/stealing and disingenuous "protect your account" campaigns for willingly turning over users phone numbers.

It is very convenient for your friends/family to be able to get your phone number from your facebook profile.

I have used it a few times to contact people for whom immediate contact was preferable to facebook post/message.

I have my phone number and mailing address on my Facebook, set to friends only.

Why not? When I grew up, we had a phone book listing everyone’s name and address and phone number so you could find them to contact.

I consider all of this essentially public information and would rather make it easier for people I know to contact me. If it gets lost in a breach, whatever. I already get plenty of junk mail because I give to charities and they sell my address.

In addition to other reasons already given, I'm not sure if this is still the case because I haven't used Facebook in a while, but back in the day, the only options for 2FA were code generator in the Facebook app itself or SMS. So if you didn't want to have the Facebook app on your phone, giving them a phone number was the only way to enable 2FA.

Your comment is disingenuous. Facebook has been around for nearly two decades. There's plenty of time there for people to make mistakes and try to fix them. Unfortunately they're competing with their contacts who re-add those same mistakes and also competing with Facebook's own incentives to not delete data.

It'll be fun if/when any of these numbers can prove they requested Facebook to delete their data under GDPR.

I believe their app uploads the address books of other people and those address books might contain your phone number and other details.