WordPress.com social networking features are being added continually. If you are one of those WordPress.com users who has eagerly been awaiting a Facebook like box you can use on your blog — the waiting is over. Look at you widgets page and discover a new addition that’s bound to make you smile. Appearance > Widgets
I must have missed the announcement. Does the Facebook like box widget now operate on WordPress.com blogs without the software stripping the code out? Yes, we now have a Facebook like box widget that can be used on WordPress.com blogs. Note that the Like Box only works with Facebook Pages (and not Personal Profiles).
You can post a Facebook badge in your sidebar.
You can also enable sharing buttons on your posts, pages, and index pages.
Also note you can have Facebook like and share buttons on your posts.
You can use a Twitter widget.
You can post a twitter follow button.
You can set up Publicize and publish to your Facebook profile page.
Alternatively, you can use Networked Blogs or RSS Grafitti for posting to Facebook walls if you prefer.