It's actually such a fantastic question that when I first read it in March, I had to leave it in my bookmarks. I knew I wanted to try to answer it, but didn't quite know how.
I am not operating a web hosting company, but I am trying to imagine what I'd pay attention to if I wanted to gauge its success.
While a ROI is always a great indicator, it may not be accurate enough if I am aggressively putting money in marketing/advertising. I could remove that cost from calculations, but then the ROI will be subjective as to the amount of money being spent isn't taken into account.
I would probably be watching a few things, but mostly three things: conversion rate, acquisition cost (in an attempt to bring it down) and how long a customer stays with the company. This will give me an understanding of revenue/profit per customer. If I am able to gradually increase this number (by upselling, among other methods), that would be incredible.