British World War One-era drama "Downton Abbey" and its colorful cast of countesses, cooks and kitchen maids has taken America by storm, dominating modern social media and bringing millions of new, young viewers to the often staid world of public broadcasting.

January's second-season opener on PBS's "Masterpiece" slot drew 4.2 million U.S. viewers -- an 18 percent rise on the first season and rising to 6.3 million in replays and online views.

That's over a million more than the regular audience for award-winning "Mad Men," and on a par with the Kardashian divorce drama finale of "Kourtney and Kim Take New York."

Cut-glass accents and castles have always been a draw for American audiences. Think 2011 best picture Oscar "The King's Speech" or Helen Mirren's Oscar-winning turn as Britain's Queen Elizabeth in "The Queen."

But what sets "Downton Abbey" apart is the buzz the show is creating on new social media websites like Twitter and Facebook for a TV channel usually associated with older viewers.

PBS, enjoying its new, hip profile, expects interest to soar again when Shirley MacLaine joins the cast as the mother of American heiress Cora, who married the aristocratic but impoverished Earl of Grantham. That bit of casting was announced last week and made headlines around the globe.

"There has been a tremendous online response," said Sarah Ball, deputy editor of VanityFair.com, who live-tweets during each weekly episode.

"PBS does historically have an older audience ... but 'Masterpiece' has courted that online fan base. They organized the live tweeting, put up sideshows and extras that really appeal to fans," Ball told Reuters.

"It is interesting that a show set in 1917 happens to have such a rabid online fan base," she added.

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Rebecca Eaton, executive producer of "Masterpiece," said the "Downton Abbey" audience is the biggest for "Masterpiece" in 17 years. She hopes fans stick around for upcoming British TV imports of Dickens classics "Great Expectations" and "The Mystery of Edwin Drood" and open their pocketbooks because PBS, a nonprofit organization, relies on viewer contributions and underwriting rather than revenue from advertising.

"My hope is that 'Downton' will inspire people to contribute to their local PBS stations ... and that will eventually help us to grow our budgets so we can co-produce more programs," Eaton told Reuters.

The show, created by British Oscar-winning writer Julian Fellowes, has already won six Emmys. When it added a Golden Globe in January, Twitter said it was the most-tweeted moment from the Hollywood awards show.

Seven of the top 20 titles available for download on Amazon.com are single episodes or DVDs of "Downton Abbey," and the drama has close to 750,000 streams online -- the highest for any work ever on PBS.

Cafepress.com has a line of T-shirts featuring some of actress Maggie Smith's most withering one-liners as the Dowager Countess of Grantham, including putdowns like "Why does every day involve a fight with an American?"

Fans have set up Facebook pages for the crusty Dowager Countess, stoic gentleman's valet Mr. Bates and other characters in the drama about English aristocrats and their servants.

But not everyone shares the love. Columbia University history professor and essayist Simon Schama derided "Downton" in a Newsweek commentary last month for "serving up a steaming, silvered tureen of snobbery" which celebrates a class system that still pervades modern Britain.

Such niceties appear to have escaped, or at least been forgiven by, American viewers who are captivated by "Downton's soap opera-like plot with its classy twist.

"I don't think there is anything wrong with a more intelligent soap, with high production values ... It dares to be romantic and soapy and dramatic and sometimes melodramatic, and its is well-written and really rich visually," said Ball, who also writes humorously affectionate weekly recaps for VanityFair.com.

"'Downton' does have a kind of fun, fizzy aspect sometimes when dealing with serious issues. You never get the sense that it is taking itself too seriously," she added.

"Masterpiece" executive Eaton says "Downton" appeals because it is a colorful piece of social history, set on the cusp of huge change.

"I think that it is tremendously reassuring, because we live in a time of unresolved crises, to watch a show that not only has beautiful eye candy, gorgeous costumes and actors, but where whatever is wrong sooner or later gets resolved," she said.

(Reporting By Jill Serjeant; Editing by Bob Tourtellotte)

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