One feature of the new build is a huge solar pv array. It doesn’t power much of the datacenter, but does reduce power requirements for some of the other buildings on site.
What this means is that this solar field is more of a token effort than a sea change. Most of Facebook's power comes from the local utility company, with a very high level of coal-based generation. Greenpeace is on the case; read its report, Facebook's First Data Center Has Bad Energy.
You can also view a panorama of the solar array, taken by tech evangelist extraordinaire Robert Scoble.