How do we calculate the number of Google+ accounts?
Before November 15, 2012, we use the numbers Google provided publicly. After that date, we began calculating the number on our own.
To achieve a tally, we first download the Google+ Sitemap :
This file itself does not contain any profile data, but it does have links to the 50,000 sub-sitemap files that do.
For each of these sub-sitemap files, we download, parse, and count the number of Google+ and Google- profiles links.
Tallying the data from all 50,000 files together gives us two numbers: the number of Google+ accounts and the number of Google- accounts.
A Google- account is a Google+ account where the G+ feature has been disabled.
We do this once a day.
At certain times however, the Google+ sitemap has been known to be unavailable or 404
to put it more technically.
You can see this in terms of data point sparsity during certain periods, for example: mid April 2013 until July 2013.
G+ recently surpassed 2^31 total users (the 32-bit unsigned integer limit), so we switched to using doubles in the graph to extend beyond this limitation. Happy 2015!