1. Log in to your Facebook account and visit video which you want to download.
2. In address bar your URL will be something like www.facebook.com change it to mobile version by replacing m in place of www and press enter.
3. Play the video after mobile version of FB gets open.
4. Right-click on video after it starts playing and navigate to copy video URL and click on it. Make sure you play video before you right-click on it otherwise you wont be able to see copy video URL option.
5. Open IDM and you will be able to see add URL option click on it and paste the video URL which you had copied.
6. Click on OK.
7. You will be able to see download file info like shown in the above screenshot, choose where you want to save video and click on start download.
8. Your download will begin and soon downloading of video will complete.