Facebook ads are their own special kind of animal. The thing with Facebook ads is that people are on Facebook with very little intent to see or respond to ads. If they do click on ads or respond to them they have made a drastic switch from surfing social media to conducting business and for some that can mean low conversion rates.
The tradeoff though is that when done right, you can get a lot of leads for cheap on Facebook. But as the saying goes, you get what you pay for.
There are exceptions but I feel it really depends on what you are pushing on Facebook. There are more options to target people on Facebook than most would care to shake a stick at and that’s what makes it an appealing forum to advertise on.