Not mine, but my wife signed up for Nextdoor several years ago. They send a postcard verification of address like Google Maps does for businesses.
She found out that all of our neighbors got a postcard inviting them to Nextdoor and it said something like "[So and So] just joined Nextdoor and invites you to join!"
She never opted into this knowingly but I'm assuming it was in the terms somewhere. I thought it was brilliant for acquisition; maybe one of the best strategies I've seen.