I'd love for StumbleUpon to be added.
We have been refactoring the code, as you can see here: https://github.com/rtCamp/rtsocial/tree/refactor. However, we are a little tied up at the moment and have limited ourselves to bug fixes for a while. If you have some php skills that you'd like to try out, you could have a look at the code and see if you could contribute.
We are hoping to add the functionality by which, users should be able to define additional buttons to the default ones. While at it, we want to add a few more options as well as twitter hash-tags.
It'll be great if you could help us out in some way, maybe with more feedback and ideally with some code! (on GitHub)
@Saurabh Of course, bug fixes first. I don't know enough code to help out with the plugin -- sorry.