Andrew Chen has a great post explaining why sticky sites that get lots of organic traffic monetize very poorly (i.e., have very low eCPMs). Here's the meat of his argument:
For the people who are curious, this is the easiest to monetize:
One-hit wonder site that exist in a particular category, are based in the US, and have lots of search traffic
In particular, your site is likely to have high CTRs since people are in a "transactional" mode. If you have all of those, and have a ton of pageviews, then you'll make a ton of money.
The hardest to monetize?
Highly sticky sites that are general (like communication), based 100% outside of the US/Europe/Japan, with lots of pageviews
In a setup like this, not only are people unlikely to want to buy anything, even if they did, there'd be no way to make money off of this group.
Before you rush off and build an SEO/SEM-driven transaction engine, though, beware that the Lord Google giveth traffic, and the Lord Google also taketh away traffic - when algorithms change, search indexes are rebuilt or somebody else can afford to lose a lot more money on buying traffic. Or maybe just because Google deemed it so. Highest RPVs (revenues per visit) and eCPMs are achieved when the site can capture the user's intent, fairly specifically and usually through a search engine, and yet offer a differentiated enough experience that the user, the search engine and whoever's dipping into the user's wallet all agree that the site added real value in the middle.
Thus, a site that buys mortgage keywords on Google merely to land visitors on a landing page full of Adsense mortgage ads and hopes to make money on the spread might be profitable in the short term but will find its arbitrage margins squeezed out soon enough. Conversely, if the site provides, say, a unique vertical search experience specific to a domain, it will be relatively immune to efforts by Google to bypass it.
Kayak.com,, a vertical search engine for travel, is a good example - visitors come with intent to find and buy travel, and they can't find the same search experience on a horizontal search engine like Google. Another example is like.com (portfolio company), that provides visual search for shopping for soft goods like clothing, shoes, handbags and jewelry. If you search for "red strappy shoes" on Google, here's the landing page you get if you click on the link from like.com. Yup, those are red strappy shoes all right. Being focused on a domain and having unique technology to look inside pictures, like.com can offer real value to the end-user, making them happy, which makes the Big G happy as well. And, oh yeah, the user who gets what they're looking for is likely to buy those red strappy shoes, which makes the merchant happy too!