If the artist was precise (if you look at the coverage of the exhibition you will find numbers ranging from 1 to 3M, both of which are significantly too low [0]), it would take about 50 more rooms (there are about 5M new photos uploaded to Flickr/day vs. 250M to FB [1]).

[0] Rough extrapolation based on http://royal.pingdom.com/2011/01/12/internet-2010-in-numbers... and a 20% growth rate http://www.kullin.net/2010/09/flickr-5-billion-photos/

[1] http://www.quora.com/How-many-photos-are-uploaded-to-Faceboo...