Not that most advertisers need convincing, but here’s yet more anecdotal evidence that trying to reach the users of social networks like Facebook and MySpace is…difficult.

Social-network users are less likely to click on an ad, and less likely to buy something, than Web users in general, says analyst shop IDC: 79 percent of Web surfers clicked on at least one ad in the past year, but only 57 percent of social-network users did.

And while 23 percent of Web surfers actually bought something during that period, social networkers kept their credit cards (or their parents’) in their pockets: Just 11 percent made a purchase.

This isn’t breaking news. But it’s still dispiriting data for social networks, which have moved past novelty stage and are under increasing pressure to translate eyeballs into dollars.

Then again, you can say that about most of the Web. And if you subtract search engine marketing (i.e., Google, and to a lesser extent, Yahoo) from the equation, marketers remain far from convinced that Web advertising works. Right around now would be a good time to change that, don’t you think?

[Image Credit: xebede]