VideoEgg's ad network, The Eggnetwork, was created to connect advertisers with user-generated content (UGC) and social networks.
Jordan Ferguson, European managing director of VideoEgg, said: "This is a strategic investment in that it allows WPP to enter the market for user-generated video content and the social network marketing space, so it's a great strategic alignment for both WPP and VideoEgg."
He added: "VideoEgg powers 20 million streams per day and is partnered with 70 websites, including 10 of the top 20 social networks in the world."
The Eggnetwork's social networks include AOL, Bebo, hi5, myYearbook.com and Tagged, with more than 12 million monthly unique users.
WPP joins existing VideoEgg investors Maveron, August Capital and First Round Capital.
The company received a £6.03m investment led by Maveron in September last year, in a second round of funding.
The company was founded in 2005 by three Yale classmates and is based in San Francisco and London.