TFA says there are "hundreds of thousands, and perhaps millions, of false positives every year." Let us assume there are a million false positives that require human intervention every year. There are 50 work weeks in the year, so to resolve 1,000,000 false positives over 50 weeks requires humans to resolve an average of 20000 per week, or 4000 per day, or 500 per hour, or 125 every 15 minutes.
Therefore, a team of 125 people could theoretically solve this problem by resolving 1 false positive every 15 minutes. At $50,000 US dollar pay scale, Facebook would need to pay out $6.25 million dollars to solve this.
Therefore, given "a user base edging toward 3 billion people is always going to generate hundreds of thousands, and perhaps millions, of false positives every year," it would cost FB ~$6.25M to hire a team of 125 people to resolve the issue, per million false positives.
That seems economical for a multi-billion dollar company.