Lol now we're talking about WMD's?
Anyway, despite the fact that I despise Assange for being responsible for the deaths of many assets who were providing intelligence for various countries, he is correct that Facebook is a spying tool. He is incorrect to limit it to just the US though, the enemy uses Facebook in such a way as well because people are dumb enough to put valuable information about themselves up there, for example, people in the Intel community will actually post their job description on Facebook. That is valuable to terrorists because it makes the Intel guy a target for counter Intel operations, like bribes, exploitation, stuff like that.
What Assange should be saying is keep personal information to a minimum on Facebook, don't post incriminating stuff, or stuff that will make you a valuable target by any adversary.