A meganiche is a specialized, i.e. not generalized web destination, that can nevertheless generate millions of users, given the global scale of the web audience.
It's a big Long Tail site.
Clay Shirky in Wired at http://www.wired.com/wired/archive/14.11/meganiche.html?pg=1&topic=meganiche&topic_set=
"I define a meganiche as a thin slice of the Web that nonetheless represents roughly a million users. The meganiche is something new, and it will have a lasting impact on online business and culture.
For most of the past decade, the basic strategy for building a successful Web site was encapsulated in the phrase "Get big, get niche, or get out." You could appeal to a broad constituency, with all the blandness and generality that implies (think Yahoo), or you could target a tightly focused group that was far smaller but easier to reach and more loyal than a mass audience (think Slashdot). Getting big would yield high volume and low margins, while getting niche would bring the inverse. Getting out was what you were forced to do if you ended up stuck somewhere between the other two approaches.
That was when 36 million people were online. Now that more than a billion people have access to the Web, there is no longer a trade-off between size and specificity. The basic math is simple: A tiny piece of an immense pie is huge. A decade ago, reaching one-tenth of 1 percent of Web users amounted to 36,000 people, a number that compared favorably with the circulation of, say, the daily newspaper in Bridgewater, New Jersey. Back then, reaching a million users required a decidedly mainstream offering (Amazon.com and MSN come to mind). Now, getting niche can be the path to getting big; one-tenth of 1 percent of today's Web audience is a million people. Forget Bridgewater – the Net is chockablock with special-interest sites and services you've never heard of but whose user base exceeds the print circulation of The Washington Post." (http://www.wired.com/wired/archive/14.11/meganiche.html?)