Download videos from Facebook is not yet offered by Official Facebook Team. But there are so many applications in playstore can do the download for you. All you have to do is, choose a best App for this download work. All Apps require your facebook login details. Keep in mind these Apps are not developed by Facebook. So providing Facebook login details may end up with risk.
These are the most popular Apps to download Facebook Videos,
Video Downloader for Facebook https://play.google.com/store/apps/details?id=com.smartapps.videodownloaderforfacebook
Video Downloader for Facebook https://play.google.com/store/apps/details?id=com.xcs.fbvideos
Video Downloader for Facebook https://play.google.com/store/apps/details?id=fb.video.downloader
EZ Video Download for Facebook https://play.google.com/store/apps/details?id=com.heyzen.vidn.fb
How to install Video downloader for Facebook
Just go to Play store from your android mobile phone. Put any one of the above titles in search box.
Click install.
After the installation, you need to give your Facebook login details to proceed further.
Once you logged in, you may find the list of Videos from facebook. Just click on the video to download it in your Android mobile.