It might make some hard core Facebook users shift to android. They will get lower engagement metrics from iOS users (the smart thing to do is to try again with moving to a web app). But the declining engagement of iOS customers will likely hurt Facebook monetarily more then the lack of fidelity with targeting.

The problem is the iOS demographic often correlates with one of the most valuable to advertise to. While targeting will become harder again, which might hurt prices, overall its not like that ad spend is going to go away. Targeting will become more coarse-grained, but at the end of the day, facebook has so many on platform data sources, that it remains quite profitable to serve them.