Nextdoor is a private social network which focuses on neighborhoods and neighborhood communication. Neighbors post about neighborhood happenings, items for sale, crime and safety issues, and other topics of neighborhood interest. Currently there are more than 600 Madison South neighbors who have joined Nextdoor.
Here are links to a couple of pages on the Nextdoor site:
This page explains more about Nextdoor, and details the Mission and Values for the site.
This is a helpful page for how to join the site.