It may be any mobile operator, there is a small bug in facebook ussd that i came across and you can use fb with no data pack and no balance indeed…
once you have to sign-in in facebook ussd by typing *325#, you may be charged for 10 rs depending on your operator..don’t worry its the ticket for lifetime usage of facebook without any data present in your phone.
Then, you have to talk with your friends with this ussd.. you will get codes for every friend you talk with.. it will look like this *325*33*01#
after few days when you need to talk with your friend using facebook ussd you can simply type the incremented keyword i.e. if you have talk with 2 friends the code you have is *325*33*02# you need to write *325*33*04# .. surprisinly you would be able to see all the facilites of facebook ussd without wasting a single rupee..
So all you have to do is sit n talk with your friend free of cost in your mobile phone