This problem is usually caused by an incompatibility issue between the Viki app and the Facebook App/Mobile Facebook. Please take the following steps to reset your Facebook session:
1. Logout from the Viki app, if logged in.
2. Delete the Viki app from your device.
3. Logout from the Facebook app if installed.
4. Uninstall the Facebook app.
5. Go to https://www.facebook.com/settings?tab=applications and remove the Viki app.
6. Visit Facebook from your mobile Safari and log out.
7. Go to your iPhone's Settings > Facebook > Logout
8. Reinstall the Facebook app from the App Store and log in.
9. Reinstall the Viki app from the App Store and try to login with Facebook.