  This paper reports the results of the deployment of Rich-State Simulated
Populations at Meta for both automated and manual testing. We use simulated
users (aka test users) to mimic user interactions and acquire state in much the
same way that real user accounts acquire state. For automated testing, we
present empirical results from deployment on the Facebook, Messenger, and
Instagram apps for iOS and Android Platforms. These apps consist of tens of
millions of lines of code, communicating with hundreds of millions of lines of
backend code, and are used by over 2 billion people every day. Our results
reveal that rich state increases average code coverage by 38\%, and endpoint
coverage by 61\%. More importantly, it also yields an average increase of 115\%
in the faults found by automated testing. The rich-state test user populations
are also deployed in a (continually evolving) Test Universe; a web-enabled
simulation platform for privacy-safe manual testing, which has been used by
over 21,000 Meta engineers since its deployment in November 2022.
