If you want to use both "Share on Facebook" and "Facebook Like" buttons in a wordpress.com blog post, just drag and drop the link below in the Bookmarks Toolbar of your Firefox browser.
Facebook Like and Share buttons for wordpress
Now read the following post to learn how to launch both of these buttons in your blog post without even leaving the blog post admin! :D
How to add both "Share on Facebook" and "Facebook like" buttons easily in a wordpress.com blog post