In the announcement, Joanna is confirmed to also be taking up the roles of Business Ambassador for the Digital Industries and Chairman of the Tech City Advisory Group. It doesn't say exactly when she will start. Quotes in the appointment from David Cameron himself as well as two ministers underline the importance of this new appointment.
Joanna Shields is Facebook's EMEA MD and a Vice President. Before that she had top jobs in Google, Bebo and AOL. Pretty awesome addition to the London tech scene, definitely something that will drive the area on both in business and in inspiration.
I first moved to Tech City in 1999 as a student (inadvertently... it was just called Old Street back then) then worked there in the early noughties for a few years and have been there again since 2007. The area has completely transformed, but something tells me there is a lot more change still to come.