Since an average server consumes about 200 Watt [1], and with an average SI EER (Site Infrastructure Energy Efficiency Ratio) of 2 [1] this translates to around 400 Watt including cooling and other overhead. In the USA, an average of 560 grams CO2 is emitted per generated Killo Watt hour [2], and this brings us to a total CO2 emission by the Facebook server park of about 59 000 ton of CO2 per year.
To put this number in perspective: The entire CO2 emission of the USA is 5 752 289 000 ton of CO2 per year and 66 693 000 ton for Finland [3]. So, this server park emits an amount of CO2 that is about 1/1000 of the total of CO2 emissions by Finland. I would argue that is not quite negligible.
As they only say that "the bulk" is running PHP (edit: for those of you to lazy to read about the Facebook architecture [1], that is solely Apache/PHP, no database, no memcache, and to quote Jeff Rotschild of Facebook: “the need for those is a function of the runtime efficiency issues of PHP” [5]), let’s assume this to be 25 000 of the 30 000 (edit: and this would be in line with other bits of info that they run around 800 dedicated memcached servers and a few thousand database servers). If C++ would have been used instead of PHP, then 22 500 servers could be powered down (assuming a conservative ratio of 10 for the efficiency of C++ versus PHP code [4]), or a reduction of 49 000 ton.
Of course, it is a bit unfair to isolate Facebook here. Their servers are only a tiny fraction of computers deployed world-wide that are interpreting PHP code.
But I think it is fair to say that using PHP, especially for large deployments, is not very Kopenhagen.