I recently happened upon Get Social Live and it works very well. It provides WP.com users with an important tool. Here is the site: http://getsociallive.com/
Have a look at the posts on my blog and you will see that there is a bar at the end of each one that has 10 buttons for Digging, Tweeting, etc.
Here's what you do:
1. Write your post as normal.
2. Put the title and the permanent post link in the boxes on the Get Social page.
3. Hit the Get Social button.
4. Copy the code generated.
5. Go back to your post and switch to HTML view.
6. Go to the bottom and add a space and then paste the code (it is long but don't worry).
7. Switch back to visual to see how it looks. I always centre the bar in visual.
Voila! Your readers can now easily send your post to 10 great social networking fora.