Contrary to the words in the introduction, that only a very small number of users become customers and end up paying, you should strive to find a way to get everybody involved (guests, users, customers) to pay in some way for their use of the service.
Typically, guests contribute eyeballs, and they pay with their time. Eyeballs can be used to get advertisers to pay for the guests and users in stead of them paying their own way.
At the same time, guests and free users can contribute because of their sheer numbers to the buzz generating engine, be harnessed to drive traffic to your site through link building efforts and to create network effects to make life harder for the competition. Each of those - make no mistake - is a form of payment. The marketing engine that a well equipped userbase represents can only be bought for significant amounts of money from specialized agencies (and they don’t work 16 hour days there, and are probably not as well informed about your product as your users are). Their presence in fora and on websites of their own, as well as the blogs, mean that you have potentially thousands of eyes, hands, and brains at your disposal. These can work for you (if you do your job well) or against you (if you don’t), so beware of the power of the users in large numbers, they’re a powertool, and like every other powertool, they have the capacity to do miracles as well as to cause great destruction.
Customers, the percentage of people that really do pay with cold hard cash (preferably monthly) are your ‘gold’, they make it possible for you to meet payroll and to pay your hosting and other costs. But they likely wouldn’t have found you if not for all those freeloaders, it’s like a distillation column with ‘unsorted’ traffic coming in at the top and multiple exits, the lowest column dripping money but each of the other stages should contribute somehow to the running of the company.
A segment that does not contribute somehow and only consumes once identified should be analyzed and somehow put to work to earn their keep.
Have them generate content, place widgets, support people view ads and so on, whatever, as long as it contributes to the whole in a positive way. From ‘freeloaders’ they’ll make the switch to ‘contributors’ and you’ll start to feel a whole lot better about them.
Especially the content generation portion can be huge, after all content generation is an expensive business and getting your end users on board to generate content can make idle hands very productive. Content that you do not need to pay for is money saved, and while not quite income, it can go a long way towards making the running of your business that much easier.