Use Conversions as the platform for getting lower cost leads.
When Facebook selects "Website Clicks" or even "Page Post Engagements" you are telling Facebook that you are looking to show your ads to people who are most likely to click to your website or engage with your post.
So this is what happens... Facebook shows your ad to people who already have a history of clicking the ad to the website or engaging in your post.
It does not necessarily mean they will sign up, and most likely they are our "happy clickers." These types are the ones we want to try to avoid.
They likely click the ads out of curiosity first, without making a solid decision on signing up. So for example if women age 24-35 are likely clicking the ad more than any other demographic Facebook will likely show your ads to them more often than anyone else.
When that happens conversion costs usually creep up and until you start to get leads, your cost per clicks may decrease, but conversion costs may increase because the people who are not signing up are clicking your ad most.
However if we use "Conversions" as our objective, then Facebook will show ads a little more evenly to everyone regardless if they have a history of clicking ads or not. Facebook does not have enough data on people in your market yet who are likely to sign up (in most cases) so it will capture the people who sign up most often first and then begin to target users in that demographic who are most likely to sign up for your services.
If sign ups is your objective, then choose conversions when you are creating your ad and create a Facebook Conversion Pixel to place on your thank you page (the actual page they land on after they sign up).