Setting up your wordpress.com blog to display sharing buttons for facebook and other social networks is quite simple, as I replied a bewildered friend, answering his email question :
1) make sure your blog is not private thru: settings/reading/site visibility
2) check with "v" the settings/reading/"Add to each article in your feed"/'sharing'
3) go to settings/sharing/ and drag facebook to the enabled services. Do the same for
each of the social networks you are interested in, as well as the options for email and print.
(be sure to differentiate between the sharing buttons you wish to appear individually and those
that you wish to be grouped under "more").
You can read more on wordpress' documentation.