Facebook — and bear with me, because I’ve somehow already written three Facebook-related stories today — is launching an interesting new service for measuring what’s popular with its 70 million unique visitors, worldwide. It’s called Lexicon (here), and it analyzes popular words that appear in semi-public “walls” (also known as message boards), on personal profile pages, group pages and event pages.

The chart collects words from across millions of wall posts each day and displays changes in word frequency on graphs, like the one below for the cult-hit movie Juno, above.

The company thinks this will be a valuable service because walls are places people pick up on what others are saying, and in turn use those same words on other blogs.

Many people already use Google Trends, which lets you see the popularity of Google search terms, to track what’s popular. If you care about what Facebook users care about, add this to your analytics arsenal.

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