Advertising group Interpublic owns approximately 0.4% of Facebook

"Advertising group Interpublic Group owns a stake in Facebook that could be valued at roughly $200 million to $300 million, The Financial Times reported.
The exact size of the stake has not been disclosed, but people close to Interpublic say it owns slightly less than a half percent of Facebook, the newspaper said in its electronic edition. Interpublic paid less than $5 million for the stake in 2006, the newspaper said.
Interpublic's stake of 0.4 percent would be worth $200 million, based on a Facebook valuation of about $50 billion, the newspaper said."
Source:  Reuters, 4th April 2011

Note - Interpublic includes Initiative, McCann, Lowe, Draftfcb etc - full list here