I do understand. I'm not necessarily advocating it. I know that the lose it updates are helping me with weight loss (down 40 lbs since Nov) and having positive reactions really are beneficial. But for fuel savings? Depends on your friends I suppose
What I was saying is that, in order to get an object on your Facebook page that shows your fuel economy, it would have to be a Facebook app. Facebook apps can do a bunch of different things, but in this case it would be a "badge" or "widget" that you could put on your Facebook page that shows the fuel economy of your vehicle. (NaNoWriMo has an app like this that shows progress toward 50,000 words.)
Apps can also post updates, or be a point to enter mileage, and may other things, but I was referring to the display aspect.