A while back, Anjali posted a quote from a product designer saying that if they had to live with one user engagement (horrible word, I know) metric for a non-transactional site or app it would be visitor recency. That's not to say that using a bundle of metrics isn't useful. It is. But since visitor recency measures what percentage of your visitors come back each day and each week, it's a good indicator of whether what you're creating is a truly great experience. One that people want to do again and again. One that stands a chance of becoming a regular part of people's lives.
It's a nice point. Eric Ries (he of the Lean Start-up movement) has talked about the trap of so called 'vanity metrics'. How every start-up (because it's so important to get talked about) will inevitably use numbers that look good on paper or on a press release (like page impressions, total users, views, message volume) but aren't action oriented.
But as Kevin Systrom of Instagram says in this interview, whilst this may be OK externally, internally it is critical that you focus on active user metrics. So for Instagram, that's not only how many people are posting per day, opening it up and checking it, but how different cohorts are using it over time (like how many of the people that first downloaded it are still using it for example). It's what Scott Cook of Intuit calls 'Love Metrics' – the kind of metrics that are better indicators of "how much people love the product, how often they come back, how delighted they are".
It sometimes seems that digital media is awash with vanity metrics. Personally, I'd much rather focus on the numbers that told me whether what I was building was something that stood a chance of being a meaningful or useful part of people's lives. I think a lot of content producers (and dareisayit media owners) still focus too much attention on their own version of vanity metrics. It's all about how many users you have rather than who they are, what they're doing or how often they come back. This, to me, is a fundamental mistake.