There are a couple of points that deserve amplification: Facebook is not the biggest operator of data centers - not by far. In fact, the new facility (which already has an expansion in the offing), is Facebook's first stand-alone. They are well known as renters of co-location space, but even considering that load, I wouldn't think they're even in the top ten of data center operators in the US.
Next, Facebook and other operators can locate in areas where utilities offer a cleaner generation mix. Claiming that network connectivity and availability of employees are deciding factors in the siting of data centers is hogwash - power costs and availability trump all other considerations, every time, with the exception of financial services companies who crave proximity to trading centers.
(Is Facebook claiming a pool of qualified employees in Prineville? Really? And superior network connectivity?)
So while I too wonder whether Greenpeace's motivations are pure (they could after all go after other high profile targets, or the industry as a whole), they have a valid point. The operators of utility-scale data centers have an obligation to operate as efficiently as possible, and to drive utilities to cleaner generation portfolios by voting with their site selection.
Looking forward to seeing how this plays out.