Well, it would be personal data about me that they store on their servers. So I could, at any point, send a GDPR request and force them to send me any data they

Well, it would be personal data about me that they store on their servers. So I could, at any point, send a GDPR request and force them to send me any data they have stored about me. Or tell them to delete it. Or sue them for having it in the first place without my written consent. Same would be true for any other "online address book" type of app. Particulary funny if people added "notes" about you to your data set. Those notes would have to be send along with all the other data they have about you, because it is data about you.