Not trying to doubt you, but was it really just a one word post that elicited that reaction? Did the message come from Facebook or someone else? Is this a common occurrence?
I'm trying to wrap my head around how the whole thing works. On one hand, mundane stuffs are monitored, on the other hand, you have stuffs like these freely discussed:
According to wikipedia, this is on the 12th most popular site in China.
I think the censors operate on very crude keyword filters. As long as you avoid the few obvious keywords, it's not that easy to get caught. The domestic web sites get around the censor by agreeing to open backdoors for the filters, yet at the same time turn a blind eye to the real disruptive stuffs that got around it.
I suspect the location of the post makes a difference. If, for example, it was posted on the wall of an anti-censorship group that is decidedly opposed to the actions of the Chinese government, then posting "freedom" there could quite easily be seen as expressing support for the group and therefore "disharmony". Probably it wasn't quite as clear-cut as that (such an overtly anti-censorship group would probably be inaccessible on the Chinese Facebook anyway), but the context probably had something to do with it; I'd be curious to hear what it was.
But it's a pretty bad sign when, seeing someone use the word "freedom", your immediate reaction is suspicion, trying to decide whether or not it shows that the speaker is an enemy. And that's how it must be for the Chinese internet police who make decisions like this one.
That level of censorship boggles the mind...