They pick you up because at least some, if not most of your friends are using app-only social networks that have access to their contacts and phone books. A few

They pick you up because at least some, if not most of your friends are using app-only social networks that have access to their contacts and phone books. A few matching entries with your phone number and they have you pegged with a very high degree of certainty and connected to more than just your phone number and text messages. Everything your friends have in their contact info for you (birthday, email, home address, work address, family members, etc.). Obviously the same thing is true for your SMS/MMS messages that are sent to friends with app-only social networks which have access to their messages and call info. In the end you're giving away most of the same data, the difference is that you're just unable to benefit from it in the same way that you could with an Android or iOS device.