The first few seconds of your video ad is extremely important.
Its main goal is to get people to watch your ad and the message you're try to pass.
In our now attention deficit world, this is extremely important.
Hook Rate : Percentage of people that's watched at least 3 secs of your video
To calculate : 3secs video view ÷ impressions
Hold Rate : Percentage of people who stayed to watch from 3secs to 15 secs.
To calculate: Thruplays ÷ 3 secs video view
Ps. These are custom metrics that can be set up on your Facebook ad manager