Personally, I feel there is only one measure that matters to us the players and that is people logged in:
|Courtesy of EVE Offline|
And quite simply put, more people means more opportunity for interactions. You got 20,000 people logged in flying around 64 player-accessible regions (ignoring wormholes for simplicity right now) and that averages to 312 pilots per region. Get 30,000 people logged in and the average goes up to 468 pilots per region, over 150 pilots to have interactions with which can represent a couple extra fleets doing something per region for you PvP types, or more buyers for you manufacturing and market trader types. Each pilot added means an increased chance of interaction with all other pilots.
So the only measure of success I think counts is getting that Period Average number in the All Time concurrent login weekly averages to rise or at least stop falling. If it continues to drop, then CCP has failed with only the degree of failure (i.e. slow decline or catastrophic collapse imminent) the outstanding question.
Do things with others, and we need others for that mantra to work. Nothing else matters.