Nextdoor is a private social network where people post messages etc.
Nextdoor promises to be another useful tool private investigators can use to locate individuals or search for comments a witness or a client might have made.
The following is taken from their website:
"Nextdoor is a private social network for your neighborhood. It's the easiest way for you and your neighbors—and only you and your neighbors—to talk online and make all of your lives better in the real world. And it's free."
https://nextdoor.com/