As if the witlesses don't also have reams of data on everyone. They do a street, they record everyone on that street. Every house and every door has a number that reaches the congregation hounders. If a person is not home, it gets logged. If someone is not interested, it gets logged. You find the tiniest of interest, that too gets logged so someone else can reach the householder at a later time (usually before the moon has had a chance to square or oppose the sign it was in during the initial call). Someone takes something, they gather that information so they can use it to scam the victim into the cancer.
Then, if someone obviously doesn't want to join, that too is logged. Someone will call back on them every year to harass them into joining the cancer, no matter how insistent the person is that they never want to be called on because they know it's a scam or because they simply wish to live their own lives without constant pressure to join a religion. Every year or so, they will be hounded into joining the cancer.
At least with Facebook or those other sites, you have to sign up before they can get a pixel of data about you.