The problem isn't just Apple's action on IDFA. It's that Facebook seems to be so poorly managed on some fronts that its reactions, rather than mitigating problems, has caused further harm. For example - in a rushed effort to get their privacy issues in order, they are deactivating the live facebook integrations of customers based on cursory/mistaken/possibly machine-based readings of their privacy policies.
They did that to us yesterday: https://shared-crater-f3a.notion.site/Facebook-is-Breaking-A...
We spend hundreds of thousands of dollars on facebook app ads per month. We can deal with IDFA giving way to more aggregated attribution (we don't want to track individuals - we just want to measure if the ads we paid for led to sales). But facebook breaking our app in production because they can't be bothered doing their job properly is very serious. It can't be solved by reducing ad spend, only by removing their SDK from our app.
If this is also happening to many other developers right now, that, more than the Q4 results or the IDFA issue itself, could be causing the drop in the share price.
In fact, if you look at the Q4 results, the earnings miss was more because of growth in G&A (which grew by 3 percentage-points of revenue if I'm not mistaken) than because of a top-line slowdown. And if you read the comments as to what made G&A grow, it's 'legal costs'.