HOW do you get a jaded young consumer to notice your ads for more than seven seconds’? Print your promos on postcards, stick them in slick racks and put the racks in bathrooms and other dark corners of the hippest gyms and restaurants. The result: patrons think they’ve discovered the freebies themselves, they grab a few cards-and even send them to their demographically desirable friends. Europeans launched the idea more than five years ago as a cheap way to reach cynical, adinundated, big spending 21- to 44-year-olds. Now the postcards are littering the United States, from New York’s Equinox gyms to L.A.’s Monkey Bar and eateries in Miami and Chicago. Top U.S. card sharks, including Fivefinger. Max Racks and Pic Nik, have nabbed clients like Calvin Klein, Wired and Sony Billboards are fine, say the firms. But you can’t mail chain letters on them.