Wikileaks only really hit the mainstream news less than a month ago. They've obviously been putting this piece together for longer than that. A runner-up article, on the other hand, can be pasted together pretty quickly.
Not to be callous, but no matter how tragic one war crime is, it doesn't rise to anywhere near the level of either the more recent Wikileaks data or, well, a lot of other news events from the year. That is to say, if the Baghdad air-strike video was the only thing Wikileaks did this year, than you wouldn't think Assange should be person of the year.
I think you're being unreasonably dismissive of a valid point: preparing a person of the year issue takes a considerable amount of time, and Assange's most noteworthy act of the year happened relatively late.
In my perception the wikileaks of iraq / afghanistan / cablegate got far more tv-news exposure throughout the year than facebook did. It is true that wikileaks itself became only an issue until nov/dec, but its leaks before that had high news value.
The events surrounding Wikileaks are the ones that are going to go on to define the next decade and influence the future of the internet. Their choice is an easy, popularist one that avoids any political controversy in the USA
I wouldn't assume malice just yet.