As iOS devices are premium, having a Lite version wouldn't serve Facebook any good. I'm pretty sure they get most of their mobile revenue from iOS devices.
Actually, Android users surpassed iPhone users on Facebook back at the end of 2012. I haven't heard that they're not displaying ads on the Lite version, so that shouldn't have an impact. That said, I've noticed that the battery drain doesn't seem to be as bad on an iPhone when compared to the drain in Android (which is why I uninstalled that app a while ago).