Hey Anh,
Alright I'm able to access the ftp now. After investigating, we are not sure why but this line of code from yootheme's template file is causing the issue.
JoomlaFolder/plugins/system/yootheme/yootheme.php http://take.ms/8riQU
We have helped you comment out this line of code and Facebook's login works fine now.
Actually is a user who experienced this issue before with yootheme.
Perhaps you should consult yootheme regarding this because this is not a proper fix.
By the way, I have also disabled these 2 modules(
http://take.ms/OGD4p) because they are causing fatal errors on the frontend. It would be best for you to consult the developers of those modules.