At any rate crating a site like a publishing business with better original content than the competition is fairly essential for anything that can get free search engine traffic. Big successes like Facebook and Youtube were money losers, living off investor funds for years before they became profitable, the finantial investment in content and paid ad traffic to make a more commercial site a success can be considerable.
Some popular sites are a fluke, like some big selling phone apps, twitter probably didn't have a massive business plan when they first experimented with it.
Investing in an established web site business might save one years of development and traffic building years, Flippa.com is the big marketplace for web businesses, just watch out for fakes traffic and revenue data.
Some people make a living out of their blog. Look at John Chow for example and there are many ways to do so.
You can sell ad space, promote affiliate offers, cpa offers, adsense, and a lot of other stuff.
I actually wrote a blog post on the top 3 ways to make money from a blog.
Feel free to check it out.
You make money in essentially three ways: 1) inserting Google AdSense PPC ads on your site, 2) promoting affiliate links, such as those from Commission Junction, Linkshare, Amazon, and others, and 3) selling a product or service. The product could be digital (downloaded not mailed) or tangible (mailed).
Of course, when using any or all of these methods, the key to making them work and generate money is TRAFFIC to your web site. Without sufficient traffic, nothing will make you money.
They still make their money from ads, it's just that they get a lot more traffic. Sometimes they sell products to generate revenue.
A blog to generate money by putting ads
Also you can create a sore to buy products to make money
