Google has recently warned against using pop ups which cover the screen of your website. As they're not very user friendly the search giant has decided that they're a negative sign with regards to the SEO of your site.
Yet people continue to use them - often to encourage an app download or to capture an email address.
I've come up against them a couple of times in the last few days with a new issue - and that's when I visit a site from with Facebook.
If you visit a website from within the Facebook app (I'm on an iPhone 6) you get a framed view of that website - with a browser bar along the top of the screen with options to return to Facebook or share the link in other ways. And very often that browser bar covers the close / X option on a pop up.
Faced with this I have a few options - I could give them my email address (but I don't), I could just leave (which seems a shame) - or I could open the link in safari and then close the pop up, which is what I tend to do. But that is more hassle than if the pop up box was just a little lower on the screen so that I could have closed it, or if I could have clicked anywhere else on the screen to have closed it.