The graph is not always increasing. Is there any metric which can give us a measure as the proxy for the number of Users in any network?
I thought unique address used so far can be a measure, but that metric is not available on blockchain.info

Is there any way to get total number of addresses used so far in the Bitcoin Blockchain. Basically I want some numeric to use as a proxy for number of users In Bitcoin.

I have seen this graph: https://blockchain.info/charts/n-unique-addresses . I see there is reduce in the number of the Unique address on many days. What would be the incentive for anyone to delete their address?