I see two practical ways out for facebook:

1. Many advertisers will revert back to using pixels instead. I wonder if you can even redirect people from an app post signup to a website with a pixel and track them that way? (or do the Apple ToSs not allow that?)

2. Alternatively Facebook need to build a tool where you can create your entire apps signups process inside facebook, so all tracking still takes place inside the facebook app, which post signup redirects via a magic link to the targeted app? So you'd basically have something like Facebook leads but built for actual signups.