Facebook has begun selling ads to resellers who specialize in "re-targeting" Web ads.
A re-targeted ad presents a product you nearly bought.
Here's an example.
You visit a retailer's Website, where you ogle a pair of boots. But you don't buy them, because you have unpaid electricity bills.
Meanwhile, the retailer has downloaded a cookie onto your tablet. In a language understood only by computers, the cookie says "she wants these boots."
The next day, the boots are pictured in an ad from the retailer on your Facebook page.
How did it happen? The re-seller's server found the cookie on your laptop and knew exactly what to do to tempt you. Or re-tempt you.
Re-targeted ads are so effective, they command three times the price of other Facebook ads, according to one reseller.
But critics say that retailers' demand for the ads is limited, despite their effectiveness.
That's because the re-targeted ads make consumers feel like retailers are stalking them.
