You're right, if provided a folder of 10 images, one of which contained a secret message then detecting it would be trivial. For Facebook to do the same with 300+ million photo uploads daily would be certainly a non-trivial process. And govt agencies don't yet have full access to non-public albums so the risk of them downloading and scanning all images is also mitigated.
If they can handle recompressing 300+ million photos a day they could proabably add a quick check for steganograpic markers and dump those images off to a separate queue for further processing, if they were so inclined.
The whole point of steganography is to not leave any markers. But yes, in theory they could scan them on upload but I'm not exactly sure what their prerogative to do so would be (unless the extension turns out to be used for terrorism that is). And plus, with that many photos to process the low false positive rate is going yo cause them huge problems.
