Buddy Media opened its European headquarters in London in July. It plans to use the funding, which was led by venture capital firm GGV Capital, to acquire other start-ups and expand worldwide.
Last year, WPP invested $5m into the group, with the aim of improving its clients' engagement with Facebook users and allowing them to integrate Facebook-targeted marketing into their campaigns.
WPP has not participated in the latest round of funding.
Buddy Media launched in the US in 2007 and claims to manage social media for eight of the world's top 10 global advertisers, with clients including Nivea, Bacardi, All Saints and Mercedes Benz.
In May, the company also bought Spinback, a start-up that measures how social networks drive ecommerce sales.
Institutional Venture Partners, Bay Partners and Insight Venture Partners were also involved in the recent funding.
Alongside WPP, Buddy Media is also backed by Softbank Capital and Greycroft Partners.