Facebook has just announced more features for group admins, such as:
The Facebook Platform Tips & Tricks: Aspect ratios for your images and videos [free template download]
When you create organic posts or Facebook ads for mobile, images and videos should cover as much screen as possible to grab attention. Also, you want to make sure your video/image is responsive on the mobile device, so the viewer has no problem watching it in its entirety no matter how they use the phone or tablet.
Choose the correct aspect ratio that best suits different placements and formats and remember to check which placements support which aspect ratios. Below are some recommended aspect ratios for different placements and formats across the Facebook platform (this includes Instagram too):
'Writing comes more easily if you have something to say.' (Sholem Asch).