It’s really dependent on the services you use as to how you can work within the limitations of Facebook to add an opt-in.
There are a couple of ways if you have a Facebook Page. They don’t allow them on your personal profile.
You can add a link to your autoresponder hosted opt-in form to the button on your page. this is a good option because FB does not display the opt-in form if it is hosted on a FB tab when viewed on mobile.
With so much mobile web use, linking to an outside form or page would be a better choice.
You can also use certain services that allow you to publish to a FB tab – but there again, you would need to redirect the page for mobile viewers so they could view the opt-in form on your hosted page.
Another option is to use Facebook’s Lead Ad (paid) service which includes a contact form in the ad.
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