That said, I must point out that the scaling issues all of these super modern "change the world" companies, such as Twitter and Facebook, have are purely for the purpose of centralizing power. It's a self-inflicted headache. It's great business to be in control of millions of users. But it's a sad state of affairs for the internet in general. We've gone from open, decentralized, and peer-to-peer protocols to one company controlling everything.
There are interesting scaling problems. But Facebook isn't one of them.