The Russians were barely able to slow down Facebook back in 2009. Unless someone on 4chan has a bigger botnet and just decided for lulz to DDoS Facebook, 4chan would not even come close to generating enough traffic to register on the #2 website on the Internet.

sure - if you found some really really expensive requests perhaps. As has been stated -- It's pretty hard to put a bump in FB's traffic.

The more likely explanation is that someone on 4chan noticed FB was down -- and then started talking about DOSing it.

Completely possible. Except the posts to call for the attack were made prior to the world-known outages. But I can't argue your point. I just found it curious that no one mentioned the planned attack.