Placement liquidity is Facebook's term for the ad slots it offers advertisers across its platforms. You can buy ads in feeds, videos, Messenger, Instagram, the mobile app, and external websites. Facebook controls the inventory and you pick where your ads show up.
There's something that's called placement liquidity in Facebook, which basically means that Facebook has a set amount of places where your ads are going to be shown. When you're scrolling on your phone, when you're scrolling on Facebook, when you're scrolling on Instagram, when you're scrolling over the web sometimes, you will see ads from Facebook. You have Messenger ads, you have feed ads, you have video ads, and these are all placements that Facebook puts towards the disposal of advertisers so that we can actually pick and choose them.
