Facebook Inc said on Monday that it made a mistake in letting Merck & Co take over a page on the social networking website from its German rival Merck KGaA.

The takeover prompted an unusual November 21 filing by Merck KGaA with a New York state court.

In it, Merck KGaA sought to force Facebook to explain how it lost the page, http://www.facebook.com/merck, and the ability to administer it to Merck & Co, a separate company.

Facebook plans to make the URL http://www.facebook.com/merck unavailable for use until both Mercks agree which company may use it. The companies may request other URLs and maintain presences on Facebook.

The transfer of the vanity URL Facebook.com/Merck from Merck KGaA to Merck & Co was due to an administrative error, Facebook said in a statement. We apologize for any inconvenience this may have caused.

Merck & Co spokesman Ronald Rogers said, We are going to continue to have a Facebook page. It is an active webpage. We are continuing to look into the matter of the vanity URL.

A Merck KGaA spokesman said, We are happy with Facebook's apology, and we are still looking into the matter.

According to court papers, Merck KGaA contracted with Facebook in March 2010 for exclusive use of the page, but discovered last month that Merck & Co was using it.

Merck KGaA said the loss of the page deprived it of an important marketing device. Merck & Co was not a defendant.

According to Merck KGaA, the Mercks became separate companies under the Treaty of Versailles in 1919. Each retained rights to the Merck trademark in different geographic areas, as part of Germany's reparations following World War I.

Merck & Co is based in Whitehouse Station, New Jersey; Merck KGaA in Darmstadt, Germany; and Facebook in Palo Alto, California.

The case is Merck KGaA v. Facebook Inc, New York State Supreme Court, New York County, No. 113215/2011.

Germany Facebook Merck & Co.