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 use a Twitter widget.
You can post a twitter follow button.
You can set up Publicize and publish to your Facebook profile page.