Most social media users operate from their mobile devices, and vertical and square format videos are the most popular videos in mobile newsfeeds.

Typical video aspect ratios are:

9×16 – Full vertical, used with Facebook and Instagram Stories, Facebook (Full Portrait), and Pinterest.
4×5 –  Used with Facebook, Instagram, and Twitter.
Used with Facebook and Pinterest.
1×1 –  Square, used with Facebook, Instagram, Pinterest, and Twitter.