This plugin works great and I enjoy the fact that it is very simple and clean.
I have one suggestion. There are so many social networking sites, and probably so many more to come in the future. I needed an icon for Reverbnation for one of the projects I was working on, and it wasn't on your list. So I used the "WordPress" slot and changed the wordpress.png to an image I could use. This "hack" works except for the tooltip on hover as it still reads WordPress.
If you made it possible to have a blank space (or several, or the ability to continue adding more slots), so that one could enter the profile name, the URL, AND the hover tooltip, then people would be able to customize their own images for whatever networking site they need.
Thanks for making a great plugin. I will rate and donate accordingly.