One public place to find up-and-coming mobile products is the top app charts that are published by Apple and Google. An iPhone app that is consistently in the top 10 of the U.S. popular app charts gets 1.5 to 2 million downloads per month, according to Oliver Lo of App Annie.

Meanwhile, Peter Farago of mobile analytics provider Flurry told me, “What makes a hit, in our view, is 1 million daily active users per platform (e.g., 1 million on iOS and 1 million on Android).”

Jack Abraham, who founded Milo and now leads local at eBay, was one of the earliest investors in Pinterest a few years ago. What got him to notice that company when so many other people couldn’t tell it was about to become a juggernaut? It was the growth chart, he recently told me.

“What I look for is 1 to 3 percent sustained growth in users per day,” Abraham said. “It could be as small as 5,000 or 10,000 users if it has that growth.”

Suhail Doshi, CEO of the widely used analytics start-up Mixpanel, said that even active user counts can hide a larger story.

“You should pay attention to what their definition of ‘active’ even means,” he said. For instance, the number of users who are active on a service within a month could be swayed by a single day’s big spike in usage. “An average rolling daily active is far more indicative,” he said.

By far the most important metric for a consumer app, Doshi argued, is retention — which is to say, the percent of users who come back the very next day after they first sign up. (Of course, measuring retention is Mixpanel’s specialty.)

“Any VC worth their salt is asking for retention numbers,” Doshi said. “You’re nothing without it.”

A reasonable retention rate is 20 to 30 percent, Doshi said. Really great retention is 50 to 60 percent.

Jack Abraham says:

“What I look for is 1 to 3 percent sustained growth in users per day,” Abraham said. “It could be as small as 5,000 or 10,000 users if it has that growth.”

It's really, really hard to tell when growth like that will sustain.

Past is not a predictor of future.

If it were easy to tell, everyone would have apps whose user numbers explode.

It could perhaps be a) revisionist history or b) he got lucky http://pandawhale.com/convo/2030/dont-eat-fortunes-cookie-michael-lewis or c) incomplete dataset -- perhaps his conversation with Liz was more extensive and finally d) some of all of the above.

What's a "sustainable" metric do you think? I find the article to be a fair and accurate insight into what investors are asking about and looking for...

Harvard MBAs / Venture Capitalists who manage multi-million dollar funds have told me, and I quote, "make your numbers go up and to the right."

It's not an exact science.

Someone who wants to invest will make up a reason why they should invest.

Someone who doesn't want to invest will make up a reason why they shouldn't.

Remember, there were plenty of people who passed on Facebook, and they have sustained user growth longer than anyone else, it appears.