Generate attention through a variety of means, both organic and inorganic
Traffic, mentions, cost per click, search results, cost of acquisition, open rate
Turn the resulting drive-by visitors into users who are somehow enrolled.
Enrollments, signups, completed onboarding process, used the service at least once, subscriptions.
Convince users to come back repeatedly, exhibiting sticky behavior.
Business outcomes (which vary by your business model: purchases, ad clicks, content creation, subscriptions, etc.)
Customer lifetime value, conversion rate, shopping cart size, click-through revenue.
Viral and word-of-mouth invitations to other potential users.