A little over 10 years ago I started a social network for neighborhoods. Instead of people joining the network, houses would join, and people proved they lived in a house by having us send them a postcard with a code on it. Incidentally, while searching for a domain, I even tried to track down and buy "nextdoor.com," which I learned a year or so later had been in stealth mode.
I first did a small launch in my own neighborhood to tune the product before going broad. It was during this phase that I discovered the toxicity of social networks. I was either a witness to, or drawn into, every petty bickering match on my side of my zip code. I am certain my product gave a wider voice to the wrong people. I should have known; ten years earlier I was an officer of my homeowners association, and it was the same thing, but face-to-face.
This wasn't the only reason I shut down the project, but it was the biggest. I thought I'd be bringing people together. I was right, but I had incorrectly assumed that doing so would be a good thing.