Facebook, according to some accounts, is growing at either 200,000 or 250,000 new users a day. Thanks to Microsoft and its $240 million investment, Facebook can now go out and but a server or two, a load balancer, and some other stuff to meet the new traffic demands. Keep your eyes and ears open over the next couple of months for some serious performance problems. I predict there will be some outages, sluggishness, blinky apps, and more.
No engineer am I, but this type of growth and scalability is not easily done. Plus, now that Microsoft is on board, a good bit of Facebook management time will likely be diverted to non-infrastructure issues. Sit tight and stay tuned.