Nextdoor

 Nextdoor  is a semi-private social network silo for sharing notes, photos, events, polls, and urgent updates with one's neighbors.

It uses a geofence of one's general neighborhood (often pre-defined) to make a semi-private social network of neighbors for sharing (semi-) private posts. Semi-private because one must be a member of a particular neighborhood to connect with other neighbors. Messages can be shared with a variety of additional surrounding neighborhoods if desired.

People generally use it to share neighborhood information and events, know their neighbors better, look for service worker recommendations (babysitters, handymen, plumbers, etc.), and act as a neighborhood watch of sorts.