Save a video to your iPhone or Android device:
Downloading videos directly from facebook is not possible, but there are number of third party apps through which you can save videos in your mobile camera roll. Friendly for Facebook is a app that works both for android device or iPhone. Here is a process to download video via this app.
Download and Install Friendly for facebook app.
Open the app and link it with Facebook.
Log in with your regular Facebook credentials.
After login, you can use this app instead of official app. It will look same as you are using the Facebook app.
In the drop down menu, you will see a cloud shape button. If you want to download facebook videos, click this cloud shape button.
Videos will download in your camera roll.