Beware of a gift scam that promises to reward a limited number of respondents with a $400 voucher to Australian retailer Woolworths. The scam typically circulates via Facebook, after a user shares a link to a "Get a Free $400 Woolworths Voucher Now" page with their Facebook friends.

Interestingly, clicking on the included link -- woolworthsfree.net -- dumps most people onto a Google search page, with no further offers being forthcoming.

According to Australia-based software architect Troy Hunt, that's because the scam uses JavaScript to identify the country that a user is located in, and then discards anyone who's not located in Australia, Albania, Canada, New Zealand or South Africa. Sister scams operating in other countries, meanwhile, include one that targets Costco users in the United States with vouchers and another that offers a $100 "free Starbucks Christmas voucher."

Don't feel left out if you can't click through; attackers are just trying to increase their odds of success. "One thing this scam does right up front is detects your location and determines whether you're likely to be sucked in by a Woolies scam or not," said Hunt in a blog post that analyzes how these scams work, as well as what the criminals behind them are seeking.

[ Learn more about Facebook security. See Facebook Adopts Secure Web Pages By Default. ]

Of course, criminals continue to launch scams -- such as cold-calling consumers and selling them fake antivirus -- because they work. "Recently I wrote about the mechanics of another Facebook scam where the 'bait' was photos of a salacious school girl. Many people -- including female friends and my mother in law -- readily fell for that one," said Hunt.

But even in countries where people can click on the Woolworths scam, the actual "conversion rate" for criminals -- meaning, the number of people who fall victim to the scam -- is likely scant. "Yes, spam and other nasties 'work' but it's really only a very small percentage of them," said Hunt. "When the king of Nigeria dies and bequeaths you $50M but only if you can help his grieving widow shift it out of the country, there's this very, very small segment of the community which actually says 'Hey, I could be onto something here.'"

To help make scam conversion rates more successful, criminals up the ante by employing a variety of social engineering techniques, such as adding a sense of urgency to their messages, including offering a supposedly limited number of free vouchers. The scam Woolworths website page also includes fake Facebook posts with kudos from two users, which appear to have been posted within the last few minutes, as well as a note at the bottom, next to a Facebook "like" button, that says over 6 million people have "liked" the page.

Meanwhile, a script on the website page counts down the number of vouchers still remaining, apparently as other consumers are snapping them up. "Every half a second the script generates a random number that is between 0 and 5," said Hunt. "If the generated number is between 1 and 2 then the number of remaining vouchers is decremented by 1. What it means is that the rate of other people snapping up vouchers doesn't appear to be constant, which adds to the believability of the whole scam."

Ultimately, how do people fall victim to the scam? To obtain a voucher, people first need to post to a fake "Share on Facebook" link, which triggers a pop-up window that's generated using a one-time link, which allows the attackers to gauge their click-through rates. Hunt said this technique "isn't that unusual for a legitimate site as it's a means of tracking how many click-throughs come from a particular 'share,' it's just a little unusual to see it in a scam." Second, users must click on the fake "Like" button at the bottom of the page, which then may trigger one of a variety of different actions, based on the user's location, but ultimately, a variety of page redirects take users to the aldaniti.net website, "where you can go off and win a shiny new Apple toy," said Hunt. In other words, the promise of one freebie leads to a website ostensibly offering even more freebies.

Regardless, clicking on a button to enter any of the competitions offered pops up a box requesting a user's name, date of birth as well as full contact details. "Or in other words, a healthy starting point for identity theft," said Hunt. Meanwhile, filling in one information-request box can beget unending other requests for information via the same site, or sister sites.

Ultimately, Hunt said all of the websites involved appear to track back to a user named "James Smith" who's based in Albania, but using a server based in Germany. "The geographic distribution is one of the reasons why these scams are so hard for authorities to get on top of," Hunt said. "People in Australia being scammed by a guy in Albania using a server hosted in Germany. Who do the cops speak to?"