They just keep tracking your device IDs and website cookies associated with your previous account

FB tracks EVERYONE, regardless if your ever had an account

https://www.digitalinformationworld.com/2022/02/new-research-reveals-how-facebook.html

>… even people that don’t use Facebook for a wide range of reasons might end up having their activities tracked by the platform whenever they visit a site that incorporates its engagement buttons. Even sites that are relatively privacy sensitive tend to give Facebook a large amount of user data
>
>Facebook can track non-users on around 44% of the websites that they tend to visit