I agree entirely with what Corey has written. Landing pages NEED to be 100% mobile compatible. Every part of it. I was using an email pop up that would look perfect on a full browser. On a mobile device it was a total nuisance and wouldn't allow the user to easily get rid of it.
Also, with regard to advertising on Facebook, much of our traffic comes from mobile newsfeeds as opposed to desktop right hand column and desktop newfeeds. So it's important to take into account the many of your Facebook ads will be seen on mobile devices.
Google also very recently added a new "mobile friendly" line to search results, meaning you'll get a little SEO bonus if you pass that test. See that here: https://www.google.com/webmasters/tools/mobile-friendly/
And check out http://mobiletest.me/ to see what your website looks like on a mobile device.
If you'd fancy a quick chat, I have a free 15 min consultation available :) https://clarity.fm/danielbilsborough/free15