Another point to this: people who run these sorts of low user, high margin companies will never reveal it to the hacker news/tech crunch/etc. crowd - for the simple reason that it will seem like it's way too easy how they are making money and it would only invite competitors.

I'm not saying it's impossible to be extremely successful on the ad model, and there are certainly plenty of successes within Y Combinator itself which have been acquired based on free services - the only take away I want people to make from my posts is that you should consider charging people for your service because if it's good enough service and you do it with high quality, there really is no reason for you not to get paid.