Decided on this slow afternoon at work to put to the test the claims by a few on here that MPG will increase dramatically (i.e. more than 5 mpg) after the break in period. I'll admit I was very curious myself as mine has gone up only 1-2 mpg over 3,000 miles.
I went on Fuelly and downloaded the data of the first 10 2012 Focus owners I found with at least 30 fillups, meaning they're probably approaching 10,000 miles.
I made sure to start with the first fill up they entered, which I assume is very soon after they bought the car new. And I went up to the first 50 fill ups if they had more than 50.
Here is a graph of what I found. Pay no attention to the spaghetti, but look at the straight lines which are the linear trendlines through each set of raw data. You will see that in this relatively small sample MPG increased in the long term by an maximum of 3 mpg. The other folks whose MPG trended upward saw maybe 1-2 mpg increase.
But the majority in this sample, 6 out of 10, actually saw their MPG go DOWN over the long term, by as much as 4mpg!
Anyway, look for yourselves. I realize this is nowhere near statistically significant and will think about spending more time on this to add data sets in the near future.