Why Smart Unlimited Data Plan Users get billed additional charges for "Facebook Mobile App Unli"?

For more than a year and a half Smart Unlimited Data Plan users have been reporting getting additional charges every month for Facebook Mobile App Unli, at the rate of Php10 (after VAT). Since the subscriber is on a unlimited data plan, users have been asking why they have been charged additional data charges. Some even reported being able to get bill reversals.

Allan Cruzado, a member of TipidPC has discovered what causes this charges which he explained in the TipidPC Smart Unified Users Thread as follows:

"after 2 months of monitoring i can finally confirm kung bakit tayo nachacharge ng FB mobile app unli. yong charges mangyayari everytime na mag login tayo ng new FB account. yong apr-may billing ko walang fb mobile charges kasi di ko ginalaw yong logged in account. then on may-june period i intentionally logged in 2 accounts and i got 2 counts of fb mobile. i did the same also on my consumable postpaid plan, nacharge din ako ng 1 count of fb mobile app. 

i wanted to call smart for the last time and give them this information. unfortunately i am still out of the country and will be back next weekend maybe. i want to hear their side why we (unlidata plan subscribers) need to be charged with this and last chance for them to fix the problem if its really invalid charges before filing a formal complaint to the NTC. 

i hope everybody having the same complaint will join us on this. if you are under UnliData Plan and getting FB Mobile App Unli charges, then please join us on this complaint. hindi po Php10 lang ang pinaglalaban natin dito kundi yong miyon-milyong piso na kinita ng smart mula sa ating mga ignoranteng subscribers. imagine 1 million subs at Php 10 each, that's an easy 10 million. if we will do nothing now because its only php10 on our side then this daylight robbery by smart will just continue."

Given my own experience with two phones Smart Unlimited Data Plans this seems to be correct. In outline form this is how it appears to work.

1. When you sign-in to Facebook from any app, whether on the mobile web browser, the official Facebook app, or a third part app like Flipboard, you will get charged Php10 which is recorded in your statement as Facebook Mobile App Unli. When are you considered to have signed in? Whenever you have been asked to give your Facebook user name and password by any app or website, it counted as a Facebook sign-in.
2. If you keep the web browser of app signed-in, you can view the feed, post updates or check-in without incurring additional charges. If you keep the app signed-in you can keep using Facebook for   days, weeks or months without additional charges.
3. If you sign-out of Facebook in the web browsers or app,  or an update to the app requires a new log-in,  than you will be charged again Php10.

This looks like a glitch in Smart's billing system, which they have not fixed. Smarts billing system counts sign-in by Smart postpaid users as availing of their 24 unlimited Facebook access service. 

To avoid the additional billing for now, best I can recommend is sign-in to Facebook only while on WiFi. After signing in, you can continue to use it even while on 3G or GPRS.