While most publications have seen a decline in subscription rates over the last decade, The Economist has been able to double its circulation to 1.5 million in that time, mainly the result of some savvy marketing.Of that1.5 million, recent reports indicate that 100,000 are digital-only subscriptions, garnering approximately $12.7 million in online dollars for the 169-year-old publication. In addition, The Economist has 15,000 Kindle and Nook subscribers, which accounts for a conservative estimate of $1.8 million in revenues. That’s a total of $14.5 million in online subscription revenues alone!But getting back to that savvy marketing. An off-the-record source informed me that The Economist has made a conscientious effort in its branding to be THE publication of cosmopolitan intellectuals, and indeed, it’s hard to find a smart world-traveler or business exec who’s willing to talk trash about The Economist. Thus, aspiring smart people feel almost compelled to buy a subscription. It’s tribe marketing at its best.The Economist has also been savvy enough to capitalize on “lean-back” devices like the Kindle, Nook, and iPad, which are more friendly platforms for their long-form content. They not only know their market, but their medium. While some readers may be asking for mobile (which they provide), the majority are going to be grateful to be able to read lengthy articles at their leisure on a lean-back device.