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.
I believe it's the "number of unique bitcoin addresses used per day".
source: https://www.quandl.com/data/BCHAIN/NADDU-Bitcoin-Number-of-Unique-Bitcoin-Addresses-Used
As you already explained the number of unique addresses that were ever used in Bitcoin could only monotonically increase.
The description of the Unique Addresses Chart only says "The total number of unique addresses used on the Bitcoin blockchain.", while the German translation is just as ambiguous.
The "used" weakly implies that it is referring to the addresses that are currently in use, i.e. as Jimmy Song already said, it's likely to be the addresses that hold a balance.
