After a period of testing, the Facebook iOS application has officially allowed users to use videos (up to 7 seconds in length) to make avatars. Proceed as follows
Step 1 : Open the Facebook application and access the personal page. Then click on the avatar
Step 2 : When a new dialog box opens, select the video available in the device or record a new one. Here, we choose option 1
Step 3 : Choose a video available in your phone
Step 4 : You will be previewed the video, click Next to move to the next step
Step 5 : Choose a Thumbnail image for the video you just recorded, to avoid the case that for some reason the video does not run, this Thumbnail image will work like a normal avatar. Then, click Save to save.
Step 7 : This is the avatar using the video