When thousands of very intelligent and highly motivated people dedicate their lives to creating applications that exploit human cognitive and social systems for advertising money, it's something to be concerned about.

Attention is a zero sum resource, and humans can be manipulated into spending their time in ways that are not in their best interest.

Edit: Tristan Harris has written interestingly on this topic:

http://www.tristanharris.com/2016/05/how-technology-hijacks-...