Branding is important when it comes to social media marketing. If you are trying to use social media sites for exposure and traffic not only is it important to brand your domain name, but it is also important to brand your user name by keeping it consistent throughout all social sites.
By keeping your user name consistent people will start getting familiar with it. This will cause them to friend you as well as vote on your submissions. And most of these sites such as Reddit are based on reputation (karma points) which means the more stories you get to the front page, the more powerful your account is going to be.
So before you run out and signup for all the social sites out there, make sure you use the same user name and icon. It will just increase your brand awareness thus making it easier for you to promote stories on these sites.