The rel=”nofollow” attribute is an easy way for a website to tell search engines that the website can’t or doesn’t want to vouch for a link. The best-known use for nofollow is blog comment spam, but the mechanism is completely general. Nofollow is recommended anywhere that links can’t be vouched for. If your logs analysis program shows referrers as hyperlinks, I’d recommend using nofollow on those links. If you have a wiki that anyone on the web can edit, I’d recommend nofollow on those links until you can find a way to trust those links. In general, if you have an application that allows others to add links, web spammers will eventually find your pages and start annoying you.
Let me give an example to illustrate. There’s a domain that runs an oompa loompa dating service. Oompa-Loompas are the small folks from the original Charlie and the Chocolate Factory. I think that the dating service is just a gag; it’s a fun way that people can play around and pretend to be oompa loompas. It used to have real people leaving messages for each other. But it also lets you add a link to a webpage, so this fun service has been inundated with people trying to get links. In the picture below, notice that every comment is pretty meaningless: “Good content and very informativity! Thanks!” and “Your website has been very helpfull to me!!”. And if you mouse over the little home page icon, you see why; I’ve highlighted one below:
The fact that webspammers will find and attack a one-off application is very telling. It shows that if you run a site that lets anyone add a regular link, webspammers will eventually find your site and spam it as well.
I’d be the first to say that nofollow isn’t perfect. For example, plenty of people will set their bots loose, and those bots will spam for links without checking if a particular page has nofollow. But the people that write the bots also aren’t dumb. If it doesn’t add any benefit to spam a particular software package, a smart spammer will avoid wasting the time/effort on that software.
If you run a well-known website or software package, webspam is more of an issue for you. Someone recently pointed me to this wikipedia thread, where someone asked if Google was in favor of enabling nofollow on wikis, so I wanted to give a quick reply: I do think it’s a good idea. For example, I’ve talked to a couple SEOs recently who said that they have a full-time person on their staff dedicated to scamming links from Wikipedia and wikis.
In an ideal world, nofollow would only be for untrusted links. Let’s take the example of a forum that wants to avoid linking to spam, but the same advice applies to wikis or any other web software. If an off-domain link is made by an anonymous or unauthenticated user, I’d use nofollow on that link. Once a user has done a certain number of posts/edits, or has been around for long enough to build up trust, then those nofollows could be removed and the links could be trusted. Anytime you have a user that you’d trust, there’s no need to use nofollow links.