Yahoo! is one company that uses paid inclusion to populate its search index. Not all of the listings
in Yahoo! are paid listings, however. Yahoo! combines both normally spidered sites and paid sites.
Many other search engines have staunchly avoided using paid-inclusion services — Ask.com and
Google are two of the most notable — because most users feel that paid inclusion can skew the
search results. In fact, search engines that allow only paid-inclusion listings are not likely to survive
very long, because users won’t use them.
There is a bit of a gray line between paid inclusion and PPC. That line begins at about the point where
both services are paid for. Detractors of these types of programs claim that paying for a listing — any
listing — is likely to make search returns invalid because the theory is that search engines give higher
ranking to paid-inclusion services, just as they do to PPC advertisements.

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