The About page says:
In this third EF EPI report, we have used test data from the 750,000 adults who took our English tests in 2012 to create the global country rankings, while at the same time analyzing the English proficiency trends that have emerged over the past six years (2007 to 2012), using test data from nearly five million adults.
I have no idea how reliable the data is (and I confess the slickly commercial look of the site puts me off, perhaps unfairly), but for what it’s worth, here are the results. Who knew they spoke better English in Slovenia than in Slovakia? (If, of course, they do.)