Make sure you're connected to Facebook on the same browser.
Right Click on the video then Copy link address.
Go to our GetFb.Net. Paste the link and Click Download.
Copy and Open the new generated link in a new tab. Then Click Next.
Select all and Copy all the text from new tab. Then Paste it on the Facebook Video Downloader, before clicking Download
Note: Make sure you're connected to Facebook on the same browser (in a different tab), that's important.