Facebook makes it really easy to track your cost per conversion (e.g. subscribe, signup or sale), and if you report revenue data back to them you can track and

Facebook makes it really easy to track your cost per conversion (e.g. subscribe, signup or sale), and if you report revenue data back to them you can track and optimize for revenue. The only excuse for loosing money on FB in the long-term is unsuccessful experimentation and that shouldn't be huge quantities.