
HyperLogLog in Practice: Algorithmic Engineering of Cardinality Estimation - ot
http://static.googleusercontent.com/external_content/untrusted_dlcp/research.google.com/en//pubs/archive/40671.pdf
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ot
Among other things, HyperLogLog was used in the recent computation of
Facebook's average diameter ("Four degrees of separation" [1]).

In this paper Google squeezes out every last bit of precision, using tables to
correct the bias and specialized algorithms for small cardinalities.

It is an outstanding effort, I guess they use these counters _everywhere_.

[1] <http://arxiv.org/abs/1111.4570>

