With the advent of new advertising methods, acquiring users has become more and more correlated with budget and
financial means. As a result, the sheer number of users that arrive at a service - or sign up for it - is no longer
a reliable metric for measuring its appeal.
This leaves metrics that are indicative of the user's interactions with the service, which in our case are
"Engagement" and "Retention". The former is usually defined as a small interaction with the product post-signup.
The latter, however, shows long term usage of the service, and is indicative of the fact that the users are happy
with the product, since the continue to use it.
Because of this, retention has become what is arguably the most important metric for a web/mobile service.