One of the most popular new tools is Twitter, a Web site and messaging service that allows its two-million-plus users to broadcast to their friends haiku-length updates — limited to 140 characters, as brief as a mobile-phone text message — on what they’re doing. There are other services for reporting where you’re traveling (Dopplr) or for quickly tossing online a stream of the pictures, videos or Web sites you’re looking at (Tumblr). And there are even tools that give your location. When the new iPhone, with built-in tracking, was introduced in July, one million people began using Loopt, a piece of software that automatically tells all your friends exactly where you are.