The two entrepreneurs behind Scrabulous, who are making more than $25,000 a month from their widely downloaded application, face a legal challenge from the two firms that own the rights to the Scrabble board game.
Hasbro owns the intellectual property rights to Scrabble in the US and Canada, while Mattel subsidiary JW Spear & Sons, holds them in the rest of the world.
Launched in July 2006 by Rajat and Jayant Agarwalla, Scrabulous has become one of the most popular games played by Facebook members and boasts nearly 600,000 daily active users.
Mattel UK said: "Letters have been sent to Facebook in the US regarding the Scrabulous application . . . Mattel values its intellectual property and actively protects its brands and trademarks."
A disclaimer on the Scrabulous website says: "Scrabulous is not a part, subsidiary, or venture of Hasbro or Mattel, and is not connected to them in any way whatsoever.
"We hope this will encourage people to purchase the official board game and make sure that Scrabulous remains something that our children still come into contact with despite these days of electronic games culture."
The brothers originally launched their online game as Bingobinge in August 2005, but renamed it Scrabulous the following July. The game became a hit only after it was put up on Facebook in June 2007.