For entertainment apps such as social networking, games and music, there are no specific goals to achieve, and more time spent the better. I don't know whether that applies to the app in this article, though.
Hence why I qualified my statement to "a tool designed...", as that is what the article is discussing (the wording of your comment makes me wonder if you read it at all ;P). FEIW, I also question whether "engagement" is good metric for social tools as well: you up engagement by making things critically need harder to use, which is a perverse incentive.