Rich constantly reminds us that “correlation does not imply causation,” relevant when looking at a recent NetworkWorld article talking about the decrease in spam, which concludes that botnet takedowns and improved filtering have favorably impacted the amount of spam being sent out.

Arguably, the disruption of botnets – the platform used to send most spam – has probably had a larger effect, with the downing of several large distribution networks coinciding with the start of spam’s decline in 2010.

Meh. Of course, that makes better headlines than all the various botnet chasing efforts paying off. But if you dig into Kaspersky’s research you get a different take.

Ads in legal advertising venues are not as irritating for users on the receiving end, they aren’t blocked by spam filters, and emails are sent to target audiences who have acknowledged a potential interest in the goods or services being promoted. Furthermore, when advertisers are after at least one user click, legal advertising can be considerably less costly than advertising through spam.

Based on the results from several third-party studies, we have calculated that at an average price of $150 per 1 million spam emails sent, the final CPC (cost per click, the cost of one user using the link in the message) is a minimum of $.4.45[sic]. Yet the same indicator for Facebook is just $0.10.

That means that, according to our estimates, legal advertising is more effective than spam. Our conclusion has been indirectly confirmed by the fact that the classic spam categories (such as fake luxury goods, for example) are now switching over to social networks. We have even found some IP addresses for online stores advertising on Facebook that were previously using spam.

Duh. Spam was great for marketers of ill repute because it was cheaper than any other way of reaching customers. If that changes marketers will move to the cheapest avenue. They always do – that’s just good business. So we can all pat ourselves on the back because our efforts to reduce spam have been effective, or we can thank places like Facebook that are changing the economics of mass online marketing. For now anyway.