What metrics should determine whether Path is successful?

It would be easy to look at some more obvious metrics - such as number of downloads, how active the user base is, etc. More untraditional and specific to their situation though: I'd like to know if their concept of 50 or less friends is taking off, because I feel that is the social premise of their app. How many friends does the average user invite via email? How many people join the app because of those emails? How active are the users they brought in? Is the user base growing organically (aka some sort of effective viral loop)? How many friends does the average user have? Also, I am concerned Path might not have enough points of re-engagement. Instagram has comments and likes, and I am compelled to check the app to see if I have any. Same with Twitter - I have to check to see if I have any mentions. I'd like to see metrics that show that people are either - deciding to jump into the app just to see what pictures/locations their friends are sharing. Or I'd like to see metrics that support that there are enough effective re-engagement calls that bring people in.