Im not an android user, so I'm not sure entirely...but i believe the official Facebook app (for Android/WP) is HTML, while the iOS app is native...depending how it uses it, could impact on data.
Is there an unofficial app you can try?
It'd surprise me if you used FB with any images in your news feed at all if you only used about 100Kb of data. 100Kb is around 2 images in crappy Facebook quality.
There are so many things that determine data usage. It'll never be the same every day. You have apps syncing in the background or doing automatic backups. Sending statistics, ...
If you have a smartphone you should really get a data plan that allows you to not worry about data usage at all. You shouldn't have to limit your mobile browsing (unless it's really extreme). You shouldn't have to fully kill apps or turn off your data.