If you've been running a blog for any length of time, you'll be sure to have come across the phenomena of comment spam. People pimping sex products and pr0n are easy to spot, but there is a whole breed of spammer out there who are rather more subtle. They particularly target blogs which allow them to post links which aren't marked as 'nofollow', making them valuable in the eyes of search engines.

Why does comment spam still exist?

Well, if you can get a link to your site from a blog with a lot of traffic, there is a chance some of that traffic will click the link and visit you. Link building is also an important search engine optimisation technique. The number of links that point at a site, or indeed an individual page on a site, is one of the factors that search engines use to determine how important a page is, and therefore how highly to rank it.

Won't spam filters catch everything for me?

Ask yourself 'Do spam filters catch everything in your email inbox?' and you'll know the answer. Most blogging platforms come with some form of defence against comment spam, and some systems, like Akismet, work collaboratively across swathes of blogs, but some comment spam is bound to escape the filtering.

Why do some blogs attract more comment spam than others?

Comment spam comes in two main forms - automated spam and hand-crafted spam. The former adopts a scatter-gun approach, attacking the most common blogging platforms, using computers to perform the task. Whether it is one server or a 'bot army' of slave machines, even a very low rate of success is worthwhile given the low effort involved once the spambot is started up.

The hand-crafted spam is harder to deal with. SEO and link-builders trade the addresses of 'dofollow' blogs, where gaining a backlink will aid search engine rankings. You can find hundreds of lists of them with a simple Google search

Rather like the harvesting of virtual gold in World of Warcraft, people are paid a pittance to leave comments on blogs that might add to a site's Google Juice.

Some of the more obvious signs of hand-crafted comment spam are:

Including a list of URLs in the comment which are unrelated to your blog's topic

A single unrelated URL left dangling at the end of a comment

Leaving the full name of a business instead of a real name or nickname, e.g "Hector's Lousiana Realtor"

Some more subtle tell-tale signs:

The email address given is a different name to the one left as the commenters name

A very short comment including the phrases "nice post", "great post", "great share" or "I subscribed to your blog"

Mangled spelling and/or grammar - a symptom of hammering out as many comments as quickly as possible in a foreign tongue.

Foreign looking names that are actually keywords. You can use Google Translate or Babelfish to establish whether 'Posaune kaufen' is a name, or the German for 'buy trombone'.

And even harder to spot:

Crafty posts that aim to put related keywords near to their link. For example 'I owe you a debt of gratitude for posting. It has really helped me consolidate my thoughts on the issue' from someone trying to place a link to a debt consolidation agency. Or signing a blog comment 'Alistair conditioning'.

How should you deal with it?

Delete. Delete. Delete. Although sometimes if the comment is actually good, but the link is a bit spammy or NSFW (i.e. a loan shark or dodgy videos) I'll publish the comment but take the link out, and re-label it 'Anonymous'.

So why persist with 'dofollow'?

Inept 'dofollow' comment spam sucks the fun out of blogging, so why do I persist in having 'dofollow' links on my comments? Well, I've been blogging for 8 years now, and still remember that leaving comments on blogs that I admired was a great way of drawing attention to currybetdotnet's early days.

I might be old fashioned, but I fundamentally still believe that 'nofollow' breaks the hyperlink principles of the web, and that people who take the time to contribute a well reasoned comment to my website deserve a link back to their own blog or personal website that 'counts'.

More information and other resources

If you are interested in finding out more, these links may be useful: