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My two cents
My two cents: I understand that approximate counting might be too specific to be put here (for me it isn't), but it is clearly a data structure - a counter offering get and increase methods. I don't know why Wikipedia classifies it as an algorithm.
It doesn't fit many algorithm characterizations, e.g.:
"A computation is a process whereby we proceed from initially given objects, called inputs, according to a fixed set of rules, called a program, procedure, or algorithm, through a series of steps and arrive at the end of these steps with a final result, called the output."
What is input to it? The counter is gradually increased - operations done inside the counter and outside it are interlaced. Most definitions require that an algorithm runs uninterrupted - given input it performs some steps and returns output. One could give the number of times to increase, but this defeats the purpose of the counter - to have small space requirements.