There is a rather subtle aspect of technology design that is often neglected, that is 'how design works within a whole context'. This idea is rather like thinking in terms of a design ecology rather than a design 'thing'. The truly difficult issue to address is 'the audience' or users, actual people.
Engagement is a long term value. A good user experience doesn't necessarily equate with engagement with a technology system. The basic unit of analysis for user experience is probably a strand of end-to-end goal driven interaction, whereas engagement looks at involvement with the technology system over days and weeks, ideally years. Engagement also considers performance within the technology ecosystem. While engagement isn't the same as 'product as a service', many successful technology engagement experiences are with long duration technology systems that operate as services.
I was thinking about examples of good and not so good engagement experiences. A good user experience may well be successfully paying my bin collection fee online or reviewing my phone bill online. I complete the tasks in less than 10 minutes, I feel confident the process is safe, I get feedback on progress and successful completion. But I'm not engaged with either of those experiences, I don't return out of curiosity or to tweak my preferences or add things. A good engagement experience might be my preferred use os Chrome for logging into 10+ different webmail accounts, my tweaking of the Chrome bookmarks or launch page. Engagement is evident in the whole experience of using my iPod, taking photos, curating some of my photos on Instagram, linking some of them to my Facebook, and the quick seamless experience of my Apps, particularly the email client on that device.
Jim Kalbach expands on his version of this discussion on his Wordpress blog (link).