And now we come to the bleeding edge of the social media universe: virtual worlds, those web-based, interactive, three-dimensional environments in which people assume the identities of video game-like characters in computer-generated landscapes populated by other characters operated by other people. Yikes! If you're new to all this, it might sound pretty weird, but you really have to give it a try. It's one of the coolest experiences on the social web, and as it turns out, incredibly useful. Here's what you need to know to dip your toe into the virtual pond.