Attention Economy: http://en.wikipedia.org/wiki/Attention_economy
As search engines have become the primary means for finding and accessing information on the web, high rankings in the results for certain queries have become valuable commodities, due to the ability of search engines to focus searchers' attention. Like other information systems, web search is vulnerable to pollution: "Because the Web environment contains profit seeking ventures, attention getting strategies evolve in response to search engine algorithms" (Page 1998). It is estimated that successful exploitation of such strategies, known as web spam, is a potential $4.5 billion per year business (Singhal 2004, p. 16).
Since most major search engines now rely on some form of PageRank (recursive counting of hyperlinks to a site) to determine search result rankings, a gray market in the creation and trading of hyperlinks has emerged. Participants in this market engage in a variety of practices known as link spamming, link farming, and reciprocal linking.
The economic incentives are similar to those of email spam: it costs very little to spammers to create huge numbers of links, so even a very small conversion rate (percentage of searchers who click on a spam-boosted search result) can be profitable. The costs of web spam are distributed among the search engines, which must spend tremendous amounts of money and labor on developing spam-detecting technologies, and searchers, who must spend attention on determining which search results are valid and which are spam (since the search engines are never 100% successful in keeping spam out of their indexes).
An attempt to change the economics of one kind of web spam is the "nofollow" attribute for hyperlinks, which causes search engines to ignore those links for the purposes of ranking results. The hope is that webmasters and makers of web discussion software will implement systems that automatically add the "nofollow" attribute to all hyperlinks not under a site owner's direct control. The effect would be to increase the cost of creating spam links, since spammers would only be able to create links on sites they controlled.
However, as opponents of the "nofollow" attribute point out, while this solution may make it incrementally easier for search engines to detect link spam, it does not appreciably change the incentive structure for link spammers unless 100% of existing systems are upgraded to support the standard: as long as some critical mass of spammable sites exists, link spam will continue. Furthermore, the "nofollow" attribute does nothing to combat link farming or reciprocal linking. There is also a philosophical question of whether the links of site commentators (as opposed to site owners) should be treated as "second-class," leading to the claim that the attribute "heists commentators' earned attention" (NoNoFollow.net 2005).
Another issue, similar to the issue discussed above of whether or not to consider political email campaigns as spam, is what to do about politically motivated link campaigns or Google bombs (Tatum 2005). Currently the major search engines do not treat these as web spam, but this is a decision made unilaterally by private companies. There is no opportunity for negotiation over the question of what is an appropriate use of attention expressed through hyperlinking. It remains to be seen [vague] whether a market-based approach might provide more flexible handling of these gray areas.
Reference for Wikipedia