For one of my clients, we experimented with Facebook lead ads to generate leads. When we started the ads, the FB lead ads brought down our Cost per lead (CPL) to 1/3rd the original cost (through conventional lead ads) and we were pretty happy. But then when we analysed the entire funnel, we realized that though the CPLs have gone down and we are getting pretty good scale, but the quality of leads was questionable.
Our conventional CPL was used to be around $7 with daily volume of 200 leads. And we used to convert around 18%. So effectively out CPL came out to be around $39. With FB Lead ads we scaled that volume to 400, with CPL around $4. But conversion was hardly around 4%. So effectively our CPL became $100.