According to my classmate, while Germans themselves didn't exchange the cards, *perhaps* they were the first to see the business end of it. Essentially the Germans were the best printers around. Although theres a lot of debate around it, many people hold that the first *commercially produced* paper greeting card was a Christmas card, manufactured in Germany. Germany used this printing know-how to supply other European markets, especially the UK, with Valentines cards meant for exchange. The UK began to produce their own and it wasn't long before these cards made it overseas.
Valentines Cards in the US were reportedly inspired by cards received from the UK. According to the History Channel, the first commercial valentine greeting cards produced in the United States were created in the 1840s by Esther A. Howland (1828-1904), the mother of the American Valentine.
Then naturally the auslanders took over...
According to the Greeting Card Association, Louis Prang, a German immigrant who started a small lithographic business near Boston in 1856, is generally credited with the start of the greeting card industry in America.
Another US Card company, American Greeting Cards, started in the 1900s by a young Polish immigrant, Jacob Sapirstein. Shortly after his arrival in America in 1905, Jacob moved to Cleveland, Ohio, and became a jobber of post cards - a popular item of the times - buying the cards from German manufacturers and selling them to local merchants.
Then the US government...
Also According to the American Greetings Site, "As the nation reeled from the effects of the Great Depression, Americans embraced the romance of Valentine's Day as an escape from the realities of hard times"
And finally, you should contrary to Fawlty Towers, Mention the War: "World War II dominated this era, and hundreds of thousands of valentines were sent to sweethearts fighting the war. Sapirstein's company, now known as American Greetings Publishers, created valentines that combined romance and patriotism, often depicting Uncle Sam, GIs, sailors, WACS and WAVES. On the home front, cutout and stand-up valentines with elements that moved using a metal grommet and paper tab device were all the rage -- the 1940s version of high-tech"
So the "research" is a bit dodgy but there you go, the Germans started it, the Brits exported it and the Americans pimped it out. The economy got dodgy, a fight broke out, an evil corporate propaganda machine kicked in and a multi-billion dollar industry was born.