There are many apps or system for d downloading the vedio of youtube but it is the easiest way.With this system you can download vedio in all formats.If you take a link of vedio from anywhere and put in these apps then you will be able to dowload this vedio(also from facebook).The download speed of this app is so much that cannot be express in a word.However these apps are not available in the playstore. You can download from the following link if you want to download apps.
click here to download snaptube
after install you will see like this given page
If you want to download vedio from youtube click on youtube. After clickcking on youtube you will see like this given page
If you want to download vedio from there you must touch on yellow download key
If you click on yellow dowload key you will able to download this vedio in mp3,mp4,hd and many other format.
Then click on any format on which you want.
Images are captured by me