The companies that make Scrabble are trying to shut down Scrabulous, an online version of the game which has become an instant hit on the Facebook website.

Indeed with 2.4 million weekly users worldwide (600,000 of whom play every day), Scrabulous is now Facebook’s most popular game.

Created last July by Rajat and Jayant Agarwalla, two young brothers from Kolkata (the Indian city that until 2001 was called Calcutta), Scrabulous has become an internet phenomenon. Revenue from advertising alone makes the pair, the younger of whom is still at university, $25,000 a month.

Now the world’s two largest toymakers, Hasbro, which owns the rights to Scrabble in the U.S. and Canada, and Mattel, which owns the rights elsewhere, are claiming that the Facebook electronic version infringes their copyrights and trademarks.

Lawyers’ letters have been sent to Facebook’s U.S. headquarters and to the brothers. But the brothers say they are undeterred and have announced plans for a Scrabulous championship, as well as a Facebook version of chess.

Facebook, devised by Harvard drop-out Mark Zuckerberg, now accounts for 1% of all traffic on the Internet.

(Below, a 1950s style advertising campaign by Ogilvy to promote the 2007 French National Scrabble Tournament.)