48 percent of the most popular pins on Pinterest lead to expired pages on top retailers’ websites, according to a new study by Curalate, the social curation company. Since Pinterest is the fourth largest referral traffic driver to e-commerce websites, that’s a big problem.

And it’s one that many retailers haven’t yet woken up to.

“E-commerce retailers are used to a world in which they have products for sale, put them on the site, sell out, and take them down,” Curalate founder Apu Gupta told me yesterday. “But now all the stuff that people are pinning continues to live on Pinterest long after the products are gone. And we’ve discovered that once a product gets popular, it continues to get more popular.”

During the recent Black Friday weekend, Curalate reviewed a number of top retailers’ websites. The team was shocked to find that 48 percent of their top ten most popular products on Pinterest actually led to dead links — expired pages. The pages no longer even showed the product that had originally been pinned … much less offer it for sale. So when people — interested, eager consumers — see something they like on Pinterest and want to buy it, far too often they are clicking through to the original product website only to hit a brick wall, Gupta says.

And this isn’t just a problem for one or two year old pins. Brands like H&M and Forever 21 are built on rapid inventory turns, sometimes as quick as weeks or months. Even more traditional retailers typically turn their inventory over seasonally, meaning that products pinned just a few weeks or months ago could already have expired.

So what’s a busy retailer to do?

“It starts with handling 404s better,” Gupta said, referring to the error code a webserver engine returns for a page that cannot be found. “But also, if a product sells out … don’t take it down. Leave it on the site, so at least the page is there and consumers can maybe order it … get on a waiting list.”

Even if that’s not possible, there are alternatives.

“Or the page can say say that the product is out of stock, but here are some suggestions … maybe some other things that are on Pinterest from our brand that are getting really hot.”

Technologies to make this easy for retailers are available now, but e-commerce managers need to become aware of the possibilities. That’s what Curalate does, Gupta told me: Identify the products that are resonating with consumers, and highlight them to brands.

“Images are the new currency of social engagement,” Gupta says. “But as images replace words, brands can’t understand that … you can’t do a text search for images. We’re the first to be able to do that.”

As Pinterest continues to grow — and as perhaps 70 percent of its users visit the site to look for shopping inspiration — knowing how to manage old inventory and assess consumer demand in a very visual world becomes more and more important.