Pila'a, Kauai -- Last Sunday morning, more than a dozen cars were parked along a six-foot wall built around Mark Zuckerberg's vast retreat on the northeast corner of Kauai, a small, remote Hawaiian island that's home to 70,000 people. The gate, which is almost always locked shut, was open, so you could walk right past the Facebook-blue sign that reads "PRIVATE PROPERTY Thank you for not trespassing." The lava rock wall, which Zuckerberg started building in 2016, inflamed some of his neighbors. It's built on a bluff a mile from the ocean and now stretches for nearly a mile along...