Google giveth and Google taketh away: many websites rely heavily on Google’s AdSense program for revenue. Israeli email marketer IncrediMail apparently relies so much on AdSense that, when it told investors early this morning that Google was barring it and disabling its ads, the company’s stock price opened 40 percent down. By the end of the day, the price dropped 45 percent since the news hit.

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A piece on Motley Fool uses IncrediMail to illustrate the impact Google’s hegemony can have. To get a picture of how large a business AdSense is, consider that, in total, Google generated about $4.23 billion in revenue in Q3; about $1.45 billion of that was by AdSense. 

IncrediMail didn’t say why it was kicked out of AdSense; it could be charges of click fraud. And it’s also possible that the company had other issues that spooked investors in addition to Google (NSDQ: GOOG). Still, it’s been otherwise healthy, with revenues rising 13 percent between Jan.-Sept. 2007. The company said it is in talks with Google and is also seeking alternative ad servers. But are there any that can compare? Yahoo’s (NSDQ: YHOO) ad network is available only to U.S. companies and Microsoft (NSDQ: MSFT) is only interested in working with major sites. Another of Google’s largest competitors is Baidu (NSDQ: BIDU), but that’s only in China.

Because 77 percent of AdSense revenue goes to the publishers, it will be difficult for others to mount a challenge. But Motley Fool offers this plea: “If Microsoft or Yahoo want a shot at Google AdSense - a market that is nearly as big as all of Yahoo! and twice the size of MSN’s top-line output - they will need to put the dagger between their teeth and treat an ad network as a loss leader. Even if it means offering 100 percent - or even more - for a limited time, AdSense will only get bigger without opposition.”