This question is a bit of an abstract thought experiment, so if you do not like such things I encourage you to stop reading.
I tend to have no intuition on the profit and revenue of internet companies, and being able to make close to accurate estimations helps greatly, and I think is a useful skill. In particular, lets focus on Stack Exchange Inc. How could we come up with fair estimate for their revenue?
I realize that a question similar to this has been asked before: See http://meta.stackoverflow.com/questions/55790/how-does-stack-overflow-meta-stack-overflow-etc-make-money and also see How does StackOverflow and sites like Quora make money? but these are not my questions. I am not looking for "general ideas," but rather a concrete Fermi-problem like computation.
Available Data: Here are some things I found in about 15 minutes of searching:
Employees: Looking at the team page: http://stackexchange.com/about/team there seem to be approximately 50 full time employees. This means that personal costs are at least fairly large.
Funding: I found this article: http://techcrunch.com/2011/03/08/stack-overflow/ which says that the company has received around 18 million dollars in funding.
Visits per day: On this page http://stackexchange.com/about it says that they have 1.3 million users, and 3.2 million visits per day, which would mean approximately 1.17 billion visits per year.
Jobs and Employers: From what I see, it appears that job postings, and employers searching for jobs is the major source of revenue: http://careers.stackoverflow.com/faq#employers with a yearly subscription costing 5000 dollars.
On the careers page, it says there are about 5000 candidates actively looking for a job, and 18000 others who don't mind being contacted.
(1) I never see any advertisements on Stack Exchange, is there still a way for a company to make revenue from visits alone without having ads?
(2) Is it safe to assume that job postings are the sole source of revenue for Stack Exchange? If not, what else could contribute?
(3) How could we go about estimating the revenue? Based on the amount of funding alone, what would expect?