I have to add that Sony went from 157 million PS2s to 87 million PS3s. That's a drop of 70 million units.
Nintendo went from 154 million DS's to 75 million 3DS's. That's a drop of 79 million units.
So Nintendo's 3DS drop was perfectly normal in comparison to PS3's drop. Both systems had predecessors that were never going to be caught. Both systems had massively bungled launches.