Very interesting. Richard Price reports on the Growth Hackers conference:
(...) it's important for the operational metric to be concrete, and also relatively early in the funnel of the user's experience of the site. As the funnel continues, drop-off becomes high, and you want to find a leading indicator as close as possible to the top of the funnel that you can work on to minimize the drop-off.
Richard also gives some good examples of leading indicators from well-known examples. Figuring out those leading indicators can be very difficult, so this is useful too:
[Facebook] looked at cohorts of users that became engaged, and cohorts of users that did not become engaged, and the pattern that emerged was that the engaged cohorts had hit at least 7 friends within 10 days of signing up.
So even a company the scale of Facebook had to use good old eyeballing to figure out what was going on!