Link popularity is crucial for your pages to rank well in search engines, but the value of guestbook links is questionable, at best, and potentially lethal, at worst.
You have probably been to a site that had a section called a "Guestbook." Many sites ask you to "sign their guestbook," and many of these guestbooks also permit HTML code in the guestbook comments, meaning you or I or anyone can visit guestbooks on web sites all day long and systematically create links back to our sites from hundreds of other site's guestbooks.
Naturally, some web marketers (probably the ones that think exit pop-ups are useful) think that by signing guestbooks and adding links by the hundreds they will improve their link popularity scores at search engines.
Before you get excited and do a Google search on the phrase "sign our guestbook" (1.9 million BTW) and head off like a link monkey, here's my take on the whether guestbook links are valid, ignored, or penalized, and if they have any impact on the success of a web site's link popularity.
Guestbook links are really no different than FFA (Free For All) links, if you think about it. FFA pages are pages where a link can be obtained by anyone (even a script) without human intervention, meaning no person even looks to see if the requesting site has any decent content.
Such link lists are obviously useless. Ask yourself when was the last time you went to a FFA link list to find a useful web site. How about never?
And since any site owner could do the same thing -- sign a thousand guestbooks -- how much credibility can such links truly have? None. If I run a site that sells snake oil I can spend my days signing the guestbooks of the best sites on the web and leech some link popularity from them? Nope.
The real question here is do search engines know about this scam yet, or do they count guestbook links as additional links for poplarity rankings? My hunch is that since guestbook links are not in any way an indication of content quality, then they do not matter at all.
If ANY search engine currently gives any credit or rankings impact for guestbook links, this impact is only because the engine hasn't yet figured out the guestbook trick, and soon will. In fact, since the majority of guestbooks pages have the word guestbook in the URL string, it would be absurdly easy for the search engines to simply ignore any link that appears at any URL with the letters guestbook in it.
And I'll bet you if they don't already ignore them they will soon.
My last point is more philosophical. If the reason you are seeking a link is because:
a) The link can be obtained automatically or in bulk numbers and b) You are trying to inflate links for SEO purposes, then the bottom line is it's all bullsh*t, and no matter if the engines figure it out today or next month, the tactic is based on a lie and shouldn't be done.
Eric Ward founded the Web's first service for announcing and linking Web sites back in 1994, and he still offers those services today. He's the editor of LinkAlert! and founder, NetPOST, URLwire, and LinkPlan, and also writes columns for ClickZ and Ad Age magazine. More information on Eric: http://www.ericward.com/linkalert/ or contact him at email@example.com
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