Facebook have posted a link to a tool they've built to track the popularity of different words and phrases within wall posts. It's available here as the Facebook Lexicon. Basically, type in one word to see its popularity, and then type in another after a comma to compare. I compared Olympics and Tibet and it's rather interesting.
It's quite a fun tool, and not without some commercial value to Facebook should they get it right: Yahoo have been able to track that kind of trend information for years, and it's highly valuable to some industries. (How many people are talking about a new Madonna album/film/whatever? etc) That kind of large-scale anonymised data collection is one of the big ways I see social networking sites being monetised, and I'm sure it's also the reason behind AOL's acquisition of Bebo. If they integrate chat and Bebo they'll be gathering a huge amount of information as to what the "youth of today" are talking about, as they're talking about it.