With iOS 5 it's not uncommon to sometimes see the label under many apps, including Facebook, sometimes change to read "Cleaning..." This is a feature of iOS 5 that Apple hasn't really explained, so it's caused a lot of you to write in an ask about it. Here's the deal...
Basically, that's iOS 5 deleting cache and temporary files so that when iCloud or iTunes does a backup of your iPhone, iPod touch, or iPad, that backup is smaller and faster.
Since adding iCloud and Wi-Fi sync options, it's become important to keep app data as clean as possible, so backups are as efficient as possible, so we're not left waiting around for minutes or hours getting as frustrated as possible.
And yes, this is the same reason that, under the original iOS 5.0, apps like Instapaper would sometimes lose all your locally cached articles. (Since fixed with the addition of a third storage option between documents and cache, one that's not cleaned but also not backed up.)
So, if you see "Cleaning..." pop up under Facebook or any of your other apps, don't worry. That's just iOS 5 keeping your iPhone, iPod touch, or iPad as lean and mean as possible.