Denial of service attacks against many targets are being launched all day, every day. This one received more publicity because Facebook is a high visibility company, and it affected so many users.
Coordinated attacks come from computers infected with viruses and malware that can be controlled remotely. Not to be melodramatic, but it's possible for any of us to have an infected computer that participates in an attack, and you wouldn't even know it is happening. An attacker may have millions of slave computers at their disposal, and merely has to send instructions for them to start attacking a target at the same time.
China has a large number of unprotected, infected computers, and many attacks come from there, but it's very hard to trace who is actually in control. There is some reason to suspect the Chinese in the Facebook attack, but it could have been someone in, say, Russia or even a malicious teenager in the Midwest.
Denial-of-service is now an underground business. You can hire someone to launch an attack on a target of your choice (such as a business competitor). It can be devastating for an online retailer. Denial-of-service ransom is also happening: If you ever want to get back online, pay us or else | Ars Technica