You can CERTAINLY make smart guesses about depth of engagement. How many times a day do you touch your email client? Your browser? Your IDE?
You can't make guesses about how good your product is going to be, but you can make guesses about how engaged the market would be if it's successful.
The point is about knowing the difference between a Facebook app like "Where I've Been" (which is neat, but not engaging on a daily basis) and FarmVille (where daily engagement is part of the game). Or, outside of the facebook world, you've got more passion POTENTIAL if you solve problems in the world of email than if you solve problems in the world Twitter analytics.
You can't make guesses about how good your product is going to be, but you can make guesses about how engaged the market would be if it's successful.
The point is about knowing the difference between a Facebook app like "Where I've Been" (which is neat, but not engaging on a daily basis) and FarmVille (where daily engagement is part of the game). Or, outside of the facebook world, you've got more passion POTENTIAL if you solve problems in the world of email than if you solve problems in the world Twitter analytics.