Sam is talking about the importance of watching leading indicators when there is a good bit of time between your present actions and future performance.
If you are relying on your users to drive growth, an easy-to-measure proxy of "do any users love our product so much they spontaneously tell other people to use it?" is Net Promoter Score[1], or the difference between the % of users who would recommend you, and those who would not.
There are other (perhaps non SV startup) businesses that dont rely on users to drive growth. For example, if you rely heavily on distribution arrangements or retail placement. In that case you might want to look at other metrics that are leading indicators of growth.