In mainland China, since yesterday (26th of April, 2015) some websites with the Facebook login plugin are been redirected to the site wpkg.org. A solution for Windows systems is this:
1. Copy the following line in a txt editor: C:\Windows\System32\drivers\etc\hosts
2. Paste it on the search box of your Windows system:
3. Open it with Notepad.
4. Add the following line at the bottom: 127.0.0.1 wpkg.org
then save it.
5. Clear your browser’s cache.
That it. Another more way is to just download this file: hosts
then unzip the file and paste it inside the following directory: C:\Windows\System32\drivers\etc\