For real businesses (read: those with actual business plans) the closest single metric of importance is Customer Lifetime Value. And the equation is very simple. Make your cost of customer acquisition less than your LTV and you will be making money.
For real businesses (read: those with actual business plans) the closest single metric of importance is Customer Lifetime Value. And the equation is very simple. Make your cost of customer acquisition less than your LTV and you will be making money.
I like Fred Wilson's analysis [1] of different revenue models it puts this stuff in perspective. The old "drive traffic too then harvest it with AdSense" model is losing a lot of steam.
The bigger issue I see in your comment is that sites at different stages in their lifecycle will want to measure different things. Customer lifetime value is the most important number for a mature business but early stage startups are nowhere near getting a realistic number for customer lifetime value. If I am the founder of a startup I want detailed knowledge of how users are interacting with the product I created to address a problem domain. I can get that info immediately after I start acquiring users, well before I have an idea of whether I have a viable business and what my customer lifetime value is going to be.
Really Instagram probably has a few OKMs (visits, uploads, hearts, comments, follows). All of those are important and should be tracked. In their documentation and educational videos, I've not seen Mixpanel focus on visits as a key metric. I'm not sure why.