In light of Google's Penguin update, it seems like a good time to suggest that you don't spam blog comments. Even if you're not technically spamming, and are leaving semi-thoughtful comments (but your ultimate goal is to get a link), it's very likely that the blog you're commenting on implements the nofollow attribute on comment links, which keeps the links from passing PageRank.
Don't forget that nofollow was introduced with blog comments in mind. Google put out a post in early 2005 called "Preventing Comment Spam," in which it said:
If you're a blogger (or a blog reader), you're painfully familiar with people who try to raise their own websites' search engine rankings by submitting linked blog comments like "Visit my discount pharmaceuticals site." This is called comment spam, we don't like it either, and we've been testing a new tag that blocks it. From now on, when Google sees the attribute (rel="nofollow") on hyperlinks, those links won't get any credit when we rank websites in our search results. This isn't a negative vote for the site where the comment was posted; it's just a way to make sure that spammers get no benefit from abusing public areas like blog comments, trackbacks, and referrer lists.
SEO consultant Carson Ward recently wrote a great article at SEOmoz about types of link spam to avoid. One of those was comment spam.
"If I were an engineer on a team designed to combat web spam, the very first thing I would do would be to add a classifier to blog comments," he wrote. "I would then devalue every last one. Only then would I create exceptions where blog comments would count for anything."
"Let's pretend that Google counts every link equally, regardless of where it is on the page. How much do you think 1/1809th of the link juice on a low-authority page is worth to you?" he wrote, referring to a screen cap of a spam comment on a page with 1808 other comments. "Maybe I'm missing something here, because I can't imagine spam commenting being worth anything at any price. Let's just hope you didn't build anchor text into those comments."
It may seem like common sense to many, but it's amazing how frequently comment spam occurs, even today, even on blogs that implement nofollow on comment links.
For the Bloggers
Matt Cutts put out a pretty popular blog post in 2009 about PageRank sculpting. Here's what he had to say about blog comments in that:
Q: If I run a blog and add the nofollow attribute to links left by my commenters, doesn’t that mean less PageRank flows within my site?
A: If you think about it, that’s the way that PageRank worked even before the nofollow attribute.
Q: Okay, but doesn’t this encourage me to link out less? Should I turn off comments on my blog?
A: I wouldn’t recommend closing comments in an attempt to “hoard” your PageRank. In the same way that Google trusts sites less when they link to spammy sites or bad neighborhoods, parts of our system encourage links to good sites.
Some bloggers aren't opposed to turning off comments though. We had a couple of interesting conversations with bloggers Jeremy Schoemaker and Michael Gray last year, following the Panda update. Panda was all about the quality of content on a page, and obviously blog comments can carry varying degrees of quality.
Schoemaker told us that he called a Google engineer friend and asked about this. Schoemaker said he was told that if anything, it’s “diluting the quality score of my page” by possibly diluting overall keyword density. Another factor could be comments that go through, but are clearly spam. These send signals that the page is not being well maintained.
Gray, who turned off his blog comments years ago, told us last year, "While I’m not living in the SEO world of 1999, things like keyword focus and density do play a role,” he adds. “If you’re doing your job as an SEO in 95% of the cases the keyword you are trying to rank for should be the most used word/phrase on your page. If you’ve gone to all the trouble to do that why would you now let and knucklehead with a keyboard and internet connection come by and screw that up with comments?”
Google says in its help center, "If you can't or don't want to vouch for the content of pages you link to from your site — for example, untrusted user comments or guestbook entries — you should nofollow those links. This can discourage spammers from targeting your site, and will help keep your site from inadvertently passing PageRank to bad neighborhoods on the web."
"In particular, comment spammers may decide not to target a specific content management system or blog service if they can see that untrusted links in that service are nofollowed," it says. "If you want to recognize and reward trustworthy contributors, you could decide to automatically or manually remove the nofollow attribute on links posted by members or users who have consistently made high-quality contributions over time."
As far as I can tell, nofllow hasn't done much to detract spammers, but at least it does keep you from passing PageRank to bad neighborhoods.