The only new scalability issue that Facebook has tackled is how to scale one enormous MySQL database. They've primarily done this by sharding it thousands upon

The only new scalability issue that Facebook has tackled is how to scale one enormous MySQL database. They've primarily done this by sharding it thousands upon thousands of times and putting over 10,000 memcached instances in front of it. Most everything else they've confronted was done by Amazon and Google years before Facebook had to.