I don't necessarily think you're experiencing an important flaw here.

I'd wager that the vast majority of people interacting with content related to evangelical Christianity are in the "pro" category, so people in the "anti" category are an acceptable false-positive.

For example, if I'm trying to sell something to evangelical Christians and 95% of my views are "pro" and 5% are "anti", that's still hugely valuable to me and something Facebook can charge a lot for.

Maybe FB could increase the price without the false positives, but neither they nor their advertisers care that much.