To be more precise, in their video they indicate that Facebook collects data from every user and sells this information to institutes like governments, policing orginizations and companies. So, in an effort to protect the world from Facebook they have declared war by launching the so called 'FacebookOp' and stated that Facebook will fall on the 5th of November 2011.
Sources report that this message might also be a scam. Others claim that it is real. Ambiguous as the message might may be, we have to wait until the November 5th to find out.
Here is the video released by Anonymous