Include an opt-in field (i.e. checkbox) within your landing page forms to opt users into your list. This gives visibility to your email offerings and provides a value add to customers who have already engaged with your product. It’s important not to pre-check the box (see pre-selected opt-in below)–instead allow potential subscribers to choose whether or not to opt in.
Some ad platforms will ask more of your landing pages than others, when running ads. For example, AdWords is much stricter, when compared to Facebook. AdWords typically requires that you do not have a ‘thin,’ site that is designed solely to collect leads. Because of this it might be a good idea to focus on using Facebook or Twitter, to begin with.