So it becomes pretty useless to target primarly on desktop.
It's better TARGETING mobile users, expecially with fb ads, but also on decisive ads, etc.
But they are quite different: it's hard to fill the classical opt-in box with the mail, and maybe if they're on mobile, they're doing something else, not have time to write etc..
And with fb lead ads, it's very probable that the mail for loggin in fb isnt the mail they read every day: maybe they signed up to fb very long time ago, etc.
So here's the "trick", either to bypass this apparently problem, and to have better leads: why better? Cause you have their real PRIMARLY mail.
The mail they uses in cell phone, so it's very probable they will open your mail.
Put an image instead of an opt in box, where the user have to click for subscribe, and then after clickin ot it, it AUTOMATICALLY OPENS their Mail Client stored in cell phone, with a pre-compiled mail, pre-compiled body, and directed to your autoresponder's list address for subscribing.
So the user has only to click "SEND" and you have a new fresh lead.
You can see a sample very simple here: Screenshot by Lightshot
The code to put is that:
<a href="mailto:firstname.lastname@example.org?Subject =Please%20sign%20me&body=Yes,%20sign%20me%20and%20 give%20me%20the%20FREE%20REPORT!">
And if you have a program for landing-opt in page like leadpages, etc, you have only to put
mailto:email@example.com?Subject=Pleas e%20sign%20me&body=Yes,%20sign%20me%20and%20give%2 0me%20the%20FREE%20REPORT
...Of course you have to change the string above with autoresponder mail for subscribing, just take care of %20 which is necesarry to put spaces.
With that strategy you could have til 50% of open rates.
Hope it helps.