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It shows whether the Facebook Platform is currently working well or not, providing graphs for average API response time and the number of errors, as well as developer updates and top live bugs. Now, when something isn't right with a Facebook app, developers know instantly whether it's their app that's causing trouble, or the platform itself.
While the charts aren't too precise and don't offer historical data (except for yesterday), they're good enough to see whether something is wrong. There's no need to visit the page itself; you can also receive updates about the platform via RSS.