Here is reply i racived from facebook
Thanks for writing in to Facebook. We’re sorry you are experiencing problems accessing your account. There is an ISP delivering Internet traffic to certain IP addresses in Pakistan that is misconfigured in such a way that it has the effect of exposing one user’s data to other users. This affects all web sites, not just Facebook. In the interest of protecting our users’ privacy, we are blocking the ISP until their misconfiguration is resolved. We are also reaching out to the appropriate administrators to help them expedite their fix. In addition, we are working on automated ways to more quickly detect third-party network errors like this.
You will not be able to use this ISP to access your Facebook account until their error is resolved. We encourage you to contact your Internet service provider to bring this error to their attention.
We apologize for any inconvenience this may cause.
The Facebook Team