Facebook Account unavailable Notification: while Login

I'm trying to login in my Facebook Account from last 45 Minutes, but I'm getting Error Notification From Facebook

We don't know why Is happens, but maybe there three cause to be happen this:

1. Migration to IVp6
2. Maintenance DataBases in India
3. Day by Day in India FB users Increasing so fb Doing Somthing with Database.

But I don't know whats the Reality, Some Account are Not Opening , they Get the Error 

Account unavailable
Your account is temporarily unavailable due to site maintenance. It should be available again within a few hours. We apologise for the inconvenience.

if you have any Idea give your valuable Comments....