There are so many ways to measure an online web application business, of course the most important being revenue.
Imagine you've got a board meeting coming up or a meeting with your investors, what are the factors you'd want to point towards to indicate whether you've got momentum or not? What would they want to see and hear?
I've had enough board meetings and done this enough that I have my own list but thought it would be great to tap into the expertise of the forum and see if there are KPIs or metrics or other that I haven't thought of and haven't included.
Lay out a short set of milestones tied to the key objectives of your business plan and then track and report your progress against these milestones.
For example, if your business plan says you'll achieve $1M in revenue from a product with a price of $1 and you'll need to sell 1M widgets, then lay out a plan for the ramp up in your sales volume to get to 1M widgets. Then, you can see by when you'll need to launch the new widget and you can back up and see when you'll need to hit specific dates for the various product development milestones - concept, development, test, etc.
Lay this out in a simple format and then share this with your board along with key tasks and risks for each milestone. As you proceed, you can color code each step - green for accomplished, red for at serious risk, etc., and make it easy for your board.
These milestones also becomes a great internal management tool for you, your team, and extended members of your team such as key suppliers.