Since 2 weeks ago it worked, and still works for old urls. but now for every new post the like button remain at zero, and if i press the button it blinks 1 but go back to zero immediately.
My site: http://arielmartini.com/
An old url that (still) works: http://arielmartini.com/2011-06-30-bixiga70
A new url that no longer works: http://arielmartini.com/2011-07-02-romulo
Even if I put the url here it doesn't work (try with the 2 url above - "URL to Like" field).
The problem was that i didn't had open graph tags on my page. Using this plugin fixed it. You can also analyse the page using this link.