Facebook won't allow numbers to be given, but if you read between the lines of various CppCon videos a 0.1% speedup in some of their core algorithms can make the costs go down by several hundred thousand dollars a year. A lot of that is the power bill because they don't need to run the AC is much to keep the data centers cool. (they also need to buy less computers, and less air conditioners to keep them cool, and eventually more building to keep the computers in)