Thanks for your comment! Yeah, a lot of non-direct marketers I talk to are like "Sending Emails? That's so oldschool, nobody reads them anymore and nobody is entering their email addresses on their mobiles anyway". That couldn't be farther from the truth. People need to understand that mobile devices replace common desktop setups more and more. I might publish a case study of one of my mobile campaigns about that if you're interested.

I know that the guys behind the Runtastic apps are collecting emails and offer more features in return. A non-mobile example would be League of Legends where they reward you with a free skin if you like their page on Facebook (https://www.facebook.com/leagueoflegends/app_208195102528120).

CPA = cost per acquisition aka affiliate offers. I don't hate it, it's just the fastest way to ruin a good list if you don't know how to do it right. Most people spam their email lists to make a quick buck but ruin their asset in the long run. It's better to build your own offers and use your own list to promote them. Advantages: you are in control, you know that the service / product you are offering has value and makes your customer happy, your profit margins are way higher.