If it's a phone number you don't even have to do it yourself. If any of your friends have linked their contacts entry of you to your Facebook account and have o

If it's a phone number you don't even have to do it yourself. If any of your friends have linked their contacts entry of you to your Facebook account and have one of Facebook's apps installed, Facebook can now know your phone number. Edit: They didn't have the phone number, according to the data exporter. Pretty plausible explanation here: https://news.ycombinator.com/item?id=19103490 https://news.ycombinator.com/item?id=19103490