A lot has been written about marketing metrics. Some people even make a career of "helping" you do this. I have a much simpler way to approach this. There are three tiers:
- Tier 3: Sales are going up and people like your product / service.
- Tier 2: You have coined a phrase or product catagory which is now in common use. Things like "markets are conversations" or "network appliance."
And you are in the top tier when your product name is used as a verb, "Let me google that" or what you wrote is know as "the bible of _____".
Seems a much more concise way to measure how you are doing.