Twitter headquarters. Matt Rosoff Three years ago, Twitter announced that it was moving into a deserted building in one of the worst parts of San Francisco.

Twitter made the move because the city had created a new tax-free zone in that neighborhood, known as Mid-Market.

We took a walk around the neighborhood in July 2011 to see exactly what Twitter was getting itself into. It didn't look pretty. But Twitter made the move, and a bunch of other tech companies followed. Then came the residential developers. Then the other businesses.

Three years later, the change is startling. See for yourself ...