Facebook decided to implement auto-playing video ads; here's how to disable them
As if I needed one more thing to hate about Facebook. Now they've decided that auto-playing video ads are cool. Here's how to stop them (when you can). 

Desktop
* Either load the following settings page directly: https://www.facebook.com/settings?tab=videos
* Or open the main Facebook website, click on the down arrow in the upper right corner of the screen, select settings from the context menu that opens up, and then on the settings page videos listed on the left sidebar there.
* The Auto-Play Videos setting is turned on by default. To turn it off, click on on and switch it to off.

Android
There is no way to completely disable Facebook's video ads on mobile. However, you can disable them when you're not on Wi-Fi: 
* Open Facebook, open the left sidebar, and tap "App Settings."
* Check the "Auto-Play Videos on Wi-FI Only" box.

iOS
* Go to the iOS Settings app from the home screen, and open up Facebook's settings.
* Tap the Settings option and under "Video," switch the "Auto-Play on Wi-Fi Only" option on.

src: http://www.ghacks.net/2014/04/14/disable-auto-play-videos-facebook/