By Ben Berkowitz NEW YORK (Reuters) - It all started innocuously enough with a July 13 blog post urging people to #OccupyWallStreet, as though such a thing (Twitter hashtag and all) were possible. It turns out, with enough momentum and a keen sense of how to use social media, it actually is. The Occupy movement, decentralized and leaderless, has mobilized thousands of people around the world almost exclusively via the Internet.