Please write an article about how you run these "profitable" campaigns. Seriously. It seems like you are the only person I've ever heard make this claim and I'm sure people here are dying to know the secret.
There's no "likes" to count or "engagement" to measure other than the normal conversion metrics -- how many of the clickthroughs completed whatever action you wanted them to complete on the landing page, how many eventually convert to paying customers, and is their LTV higher than their CPA.
Facebook's also a great channel for retargeting. I run a campaign for Improvely there and the ads only appear to people that have already hit the signup page within the past few days.
I for one am confused as to why people chose the other indirect, convoluted way: buying likes, directing people to their FB page, THEN hoping they click through to their actual website.