I’ve noticed that people ask this question a lot. The answer is two fold, but it’s very simple. Before you go rearranging your websites in a panic, take a look at some of the Adwords advertisements you’ve been getting recently.
In order for Google to have ads to show, they have to recruit advertisers. To do this, they have to offer them savings. Adsense publishers have no control over which advertisers Google recruits or how much (or how little) Google will charge them. So even if your site is extremely well stocked with ads in every single space allowed, your traffic is great, and your search rankings are high and you have great user feedback, you have no control over whether Google will charge less than a cent per click or $5 per click.
So, when Google is making special savings offers to Adwords clients, you will probably be paid less per click.
The other reason is ad relevancy. Even if your keywords and tags are tight, and your site has lots of well written content, some advertisers purposefully buy space on sites with nothing to do with them or even ideals counter to theirs. I often get Christian fundamentalist ads on my spirituality, Pagan, and Vodun sites, some of which are quite offensive. I’m strongly considering not posting Adsense on them because they weren’t really getting many clicks anyway.
Basically, for the non mega-site webmaster, Adsense is a great way to make your hosting costs back, but I wouldn’t count on it for more than that. To me, it’s worth it simply because it makes the sites pay for themselves, but the hit-and-miss nature of it just doesn’t make it something I would rely on for consistent site revenue.
As always, the best way to make money on the internet is to be offering something that people are willing to pay for. If you own an extreme site on the level of Squiddoo or Hubpages, another articles database or forum host or something, then Adsense would pay you a living, but otherwise, stick to good old fashioned having a product and selling it. Then you won’t worry so much about Adsense ups and downs, get frustrated, and chuck it.
Aside of the crazy religious sponsors, for most sites, Adsense is a great way to bring relevant products to your readers. I have a site about cornrows, but I don’t sell hair supplies. Some sponsors do though, so it has helped my readers to not only find the information they need at my sites, but the products as well. So when you’re deciding whether or not to keep using it, bear that in mind.
If you’re not making anything from Adsense, or what you’re making isn’t really pulling its weight at the tops of pages, you might still want to keep blocks at the bottom or sidebar. Readers like the convenience, and this does good things for your traffic.