Using Android ?

Try out this step by step guide

1. Go to facebook app
2. find out the video you want to download
3. play it
4. click on the icon with “3 dot”
5. Click “Copy video url” from the icon
6. Now leave Facebook app
7. Go to google chrome
8. paste the url you have copied from Facebook app
9. go to the url
10. play the video
11. now pause it
12. right click on it
13. click on Save Video

Done , It will start the download process