Face, which was founded in 2003 by Andrew Needham, Henry Yates and Job Muscroft, owns , a web community for 16- to 25-year-olds who share their opinions and ideas with brands and get rewarded for them.
Face specialises in helping companies such as Unilever, Google and NatWest discover insights into the UK's youth market and helps them to generate word of mouth with young people aged 11-18 and 18-30.
Andrew Needham, founding partner of Face, said: "The fit with Cello is perfect and provides Face with an exciting opportunity to grow quickly by allowing us not only to leverage off its established international research network but also its strong client relationships with major brands."
This is the latest in a series of acquisitions by Cello Group. In September last year, the firm acquired direct marketing support business Magnetic Advertising Company for as much as £4.32m and following this, in October, it acquired 2CV Limited, the specialist consumer market research consultancy, for an initial consideration of £5.4m.
Kevin Steeds, Cello chairman, said: "The Face stake is consistent with our strategy of building a substantial international research and consulting group.
"Face is an exciting and innovative company with a strong youth heritage that has developed a range of leading edge research and marketing techniques to help global brands such as Unilever collaborate, innovate and co-create with young people."