Few months ago, WordPress official repository started to show number of 'Active Installs' of plugins. How is it estimated?
For example, this plugin is downloaded 2,365 times and it shows Active Installs: 100+
Again, this plugins is downloaded 437 times only with same amount of Active Installs:!
Short answer: the update API. Whenever your WordPress install makes an update check, it'll ping wordpress.org with information about all the plugins you have, versions etc., but also whether they are currently activated or not - they then aggregate all this data from all active WordPress installs and hey presto, you get an idea of how "used" the plugin actually is.
