It’s important to note it’s not an exact number as of now, but rather a minimum count like 40+. it gets less specific the higher the number is. For example, Jetpack’s active installs are currently listed as 1,000,000+.
It has always been a slight pet peeve of mine when a download count is used as a measure of success for a theme or plugin. While far from irrelevant, active installs are a much more useful metric.
For example, if somebody installs a plugin, realizes it’s not a right fit for whatever reason, and deactivates, the download count remains unaffected. Active install count won’t.
Pretty cool move!
P.S. I haven’t seen any mention of this on the Make WordPress Plugins site, but I’m expecting that will come soon.