Seems to be popular in various parts of California where home ownership is high. It is also seems to be a haven for the "worst" type of neighbors who like to complain about everything and might be slightly racist. See "The Jerks Nextdoor" for a good overview of what happens way too often on the site: https://www.newsreview.com/sacramento/jerks-nextdoor/content...

Edit: splat brought up a good point. A lot of municipalities are using it as an "official" communications platform.