Right now I do not have an easy way for you to add custom ones. I have a plan in the future to make it more customizable so you can add things to the sidebar, but that is a ways off yet.
If you notice in my blog I use the AddThis bookmark button at the bottom of each post. If you mouse over it you will see it has share on Facebook, MySpace, Email, Bookmark etc all built in. You can enable it in your site from site settings or from the blog settings under the gear icon.
Hope it helps,