None of the codes that Facebook provides for a Likebox works in a wordpress.com blog.
A workaround is available.
The Facebook Likebox will only show up if a link is clicked, but it does then open right in your blog.
If you want a Facebook Likebox in your wordpress.com blog, you can see how this workaround looks and also learn the trick here:
How to add a Facebook Likebox in a wordpress.com blog – part 2