Does Facebook use web advertising conversion signals to optimize app advertising campaigns?

Almost certainly. But my sense is that web conversion signals don't provide much value to app advertising campaign optimization.

Following the announcement that the IDFA was being deprecated, I heard a number of people claim that mobile advertising wouldn't be impacted by the loss of a mobile advertising identifier because web conversions were just as useful as in-app conversions for optimizing mobile app advertising campaigns. The idea here is that web conversions -- such as a person clicking on an ad for a pair of shoes within the Facebook desktop web newsfeed and subsequently purchasing those shoes on the e-commerce company's website -- provide context around disposable income and purchasing proclivities, and that context can be used to optimize the ads for apps shown to a user.

This doesn't seem credible to me. First, it intuitively doesn't make sense: in-app behaviors are very specific, and in-app purchases are very rare for any given app. Yet online retail commerce generates more money than in-store retail commerce ("General Merchandise Stores" below):

This is to say: buying retail products online is very popular, but buying items within an app is still exceedingly rare. The disproportionality between these two activities would produce many false positives if using web conversion signals to optimize for app advertising campaign targeting: people that shop online but don't purchase items in apps would be shown ads for apps more often than they should.

Another reason why I don't think web conversions provide useful signal for app advertising campaigns is that Facebook built out totally unique campaign strategies for collecting in-app events to optimize app advertising campaigns: VO and AEO. Ostensibly, these campaign strategies, if not exclusively, rely primarily on in-app events for campaign optimization.

And lastly, Facebook has been very vocal about the fact that IDFA deprecation will hurt advertisers:

We are still trying to understand what these changes will look like and how they will impact us and the rest of the industry, but at the very least, it’s going to make it harder for app developers and others to grow using ads on Facebook and elsewhere. Advertising clients are asking us how to maintain their performance and we’re working on it. Our view is that Facebook and targeted ads are a lifeline for small businesses, especially in a time of COVID, and we are concerned that aggressive platform policies will cut at that lifeline at a time when it is so essential for small business growth and recovery.

If web conversion signals provided a great degree of value in optimizing app campaigns, then IDFA deprecation would have little impact on the efficiency of app advertising campaigns.

My sense is that web conversion signals are used primarily as a means of assessing a user's ability to spend money, but not for specific targeting purposes (eg. pairing App X with User Y).