For calls and SMS, make sure that your AT&T phone is
1. network unlocked
2. capable of using the Philippines GSM frequency (900 and 1800 mhz as opposed to the US 850, 1900 mhz). Many of the newer phones are now either triple band or quad band which you can use in most parts of the world.
Get a Globe prepaid sim and make sure it would have a sufficient load to avail of the promo you like. If it's just FB, the following are the rates:
Prepaid users may register to SUPERFB by texting < keyword > to 8888.Keyword Access Code Price Validity
SUPERFB10 8888 P10 1 day SUPERFB15 8888 P15 2 days SUPERFB50 8888 P50 7days SUPERFB199 8888 P199 30 days
You can also register via *143# and select SUPERFB under Surf Promos.
In addition to that, you may want to load up more than what is required by your FB use for calls and SMS.
Hope this helped.