I have been using this app from years when I had Nokia phone. Now, I have android smartphone but I still prefer this app over main Facebook app.
The size of the app is less and it is convenient to use. Even if you are using it on 2g network it works perfectly. The best thing about this app is it allows messaging where as the other Facebook app forces you to download an app called Messenger. This Messenger app consumes lots of storage and it also consumes more data.
This app gets regular updates.
If you want to save your mobile data and save the internal storage of the phone I would recommend this app over Facebook app.
