Former Netflix employee (2010-2013) here and, if there's one thing Netflix does as well or better than anyone in the industry, it's running the numbers. In particular, in those days, we had two key metrics that were strongly correlated and we would attempt to drive up: Streaming hours and account retention. Higher usage was strongly correlated with account retention to the point that they were the core of nearly every experiment we did.
I think these are reasonable numbers to focus on, but other relevant variables could be just too hard to quantify or set as goals...
For instance how Netflix's catalog is attractive to new users/markets can be checked in regular polls, but it would be way more difficult to follow with fine granularity, far less precise, and ultimately a harder to handle number than just retention or number of new accounts.
This means Netflix could see decent growth on its numbers, good retention and a steady flow of new accounts created, while struggling to reach new markets where competitors are doing great.
This is an extreme example, but Blackberry typically had very good user retention and users loved their devices. Looking only at these numbers, they were doing fine for a long time (which is nothing to sneeze at)
