There is an ongoing (and ultimately un-winnable) discussion on whether to watermark your online images or not.
From an aesthetic point of view, watermarks can be ugly. Many photographers plaster large logos or text on a piece with no regard for how it looks.
Watermarks are also very inefficient at their stated purpose of limiting photo theft. If someone wants to steal your photos, they will crop out, retouch, or just ignore your watermark.
Whenever I upload an image, especially to something like Facebook, I put my name and website on the image. I figure that it makes it easier for someone to find out who I am, and possibly see more of my images.
I’m not sure if it works, but for the amount of time it takes, I figure it can’t hurt too much, as long as I try to keep the watermark classy…