iPhone users are probably among Facebook's most valuable advertising demographic (ie. lots of disposable income). Apple's decisions to block tracking are hurting them in a big way, and offering a paid subscription probably won't seem attractive to users if Facebook prices it at the full value lost.

We've trained people that everything is free. We can't get away from that now that the genie is out of the bottle. Furthermore, people might just decide Facebook isn't worth it for them at the level of functionality they provide. As more churn happens, the overall value of the network decreases.

Apple just checkmated Facebook.