The “freemium” business model is now the growing global default for classified ad business models — where the ad is free but the advertiser can pay a small fee for highlighting it, promoting it, extending it or otherwise making it more prominent.

Enter Facebook.

Just a few days before its initial public offering, Facebook is testing a freemium model for classifieds and other “highlighted posts.”

(We learned about it from Mashable.com, which in turn learned about it from the New Zealand blog “Stuff.” We haven’t yet had a chance to get confirmation from Facebook or follow up. We’ll do that next week, but Facebook is in a “quiet period” under securities laws and we don’t know whether they’ll talk at all.)

Facebook told Mashable “highlighted posts” is in a limited test, with prices ranging from nothing to “a couple of bucks. Two examples of potential “highlight” users the company gave were someone selling a car, or a band promoting an appearance. Mashable said highlighted posts could make Facebook a competitor to Craigslist. (Well, isn’t just about everybody?)

The Mashable piece includes a screenshot of one example of a purchase screen.

Monetize, monetize, monetize. We hate the word, but it’s something Facebook had better be doing. Because once it goes public, the pressure for increased revenue will be relentless.