If you've been on Facebook even somewhat regularly over the past few years you've probably seen the ad. It claims to for a sale on Ray-Ban sunglasses. You may have even seen your friends post it. So what's behind that weird ad?

It turns out, nothing good. According to the Washington Post, the ad leads to a two-pronged attack on you and your friends' information.

Here's how it works. That ad leads to a fake online store, claiming to sell Ray-Ban sunglasses. If you purchase anything there it steals your credit card information. To add insult to injury it will also try to get your password via malware or more traditional means. Once they have it they'll use your account to spread more spam to your friends, who may trust things that you post.

Facebook and Ray-Ban are working to get rid of the ads. But a few will slip through the crack. Here's something very simple you can do to protect yourself: Whenever you see one of those ads for coupons or sales from well-known brands, just check on the brand's Facebook page to see if they've also posted about the sale. If you can't find it on there, that ad was probably a scam.