Ok, now it's working, I'm not sure exactly what fixed it, but it must be one of these options or all of them together:
I've changed the OpenGraph type from url to article.
I've set the base domain in the Applications Settings page in Facebook to blogspot.com
The other posibility is that it was working all along, but it didn't refreshed the buttons that I've already clicked, of the ones that were already there after the change that actually worked, because when I posted a new post I've realized that for the new one it was working.
Anyway, following this tutorial should help:
Harry Potel
2010-08-03 14:43:38