How can the number of unique addresses decline? Blockchain.info has various charts over time, including one for "unique addresses". They do not explain what it means, but presumably it is the number of unique addresses that appear in all transactions in the blockchain.

If this is true, then I would expect the number of unique addresses to keep going up indefinitely. How could the number of unique address decline?
Most likely, it's the number of unique addresses that hold a balance, not have appeared on the blockchain.