Website custom audiences are shrinking due to iOS opt-outs. But, Facebook could replace these audiences so easily. There are limitless audiences that Facebook could create today that would help advertisers.
The key: These audiences wouldn't be impacted in any way by iOS opt-outs.
Read more here:
https://www.jonloomer.com/audience-targeting-options-facebook-needs-in-the-age-of-less-tracking/
