@Gargron Well... metrics are important but what you described is more like b-2-c POV, to get users to buy/spend money on products they are offered. Those things are more for marketing (and advertising?) to measure website effectivity to influence a user. Placing a simple tracking (ie. GA or similar) to see if onboarding works or not ie. if joinmastodon feeds signups or not.
UX is a discipline to make our sites to communicate our message effectively, to make our user return to use our service.