Don't know what Lexicon was? That's OK, a lot of people didn't; it was announced very quietly, but it was a neat tool. You could use Lexicon to see how frequently, and when, certain terms were trending in status updates, just like you can use Google Trends to do the same with search terms.
For example, our own Stan Schroeder was a bit perplexed a couple years back when the word "sex" had a massive spike in usage on May 30, 2008. It was the launch day of the Sex and the City movie in the United States — understandably something a man in another hemisphere might not follow closely.
Unfortunately, those amusing little mysteries and insights are now lost to Facebook users, and Facebook hasn't provided a satisfactory explanation as to why. The notice that hangs where Lexicon used to be says:
"Thank you for your interest in Lexicon. We are removing the Lexicon product from Facebook for the time being. We may bring components of Lexicon back in the future, but we are focusing development on our analytics tools for Page owners, advertisers and Platform developers."