Downloading a Facebook video using the Facebook app on your smartphone might not be possible.
Here are practical steps on how to download a Facebook video using your smart phone .
1. Open your browser.
2. Input www.Facebook.com as the url
3. Log in to your Facebook account
4. locate the video you want to download
5. Open the video
6. long press the video, a number of option will be given
7. Select the download option.
And you have your video.