Chris Norton, who has been at Blue Rubicon since 2008, will join Facebook on 4 January.
He said: "Blue Rubicon is an outstanding agency full of exceptional people and great clients; I have been so lucky to play a part in its success and growth over the past seven years.
"Leaving such a talented team is very hard. Facebook is a fascinating company that is changing the way people connect with each other. This really is a dream job that only comes up once in your career."
Blue Rubicon chief executive Gordon Tempest-Hay said: "Chris has been an important part of the firm’s success over the past seven years. While we’re sad to see him go, we’re delighted that he is moving to become one of our clients and that we will continue to work together for many years to come."
Blue Rubicon has handled PR for Facebook for five years, including corporate work previously held by Edelman which it won last year.
Facebook's current EMEA comms chief is Stefano Hesse. He will continue at the company as part of Norton's team.