One experiment involves offering a subscription service to online premium study notes.  We measure number of subscribers, downloads, and revenue.  Students who subscribe typically keep and use the subscription until they finish school: the biggest problem is the very low conversion rate in the first place.  We know that students want study notes, because they download free notes from our site all the time, but very few are willing&#x2F;able to pay.  This is a recurring theme for us: we get enormous interest when offering something for free (whether notes or seminars), and get complaints as soon as a price tag is attached.<p>We reached the point a couple of years ago where we realised we had largely captured the maximum potential user base in our home state, which left two obvious strategies: growing revenue (in that state) and&#x2F;or growing the user base (in other states).  From that point our core metric has been revenue, with a view to further growth once we had nailed the model.  Our primary revenue source has been advertising (from universities and colleges and others), as that seemed to give the greatest return for the fewest complaints.  It grew very quickly from zero, but has stagnated in recent years as advertisers developed a preference for programmatic platforms.  That shift has prompted the present search for a new business model.  Meanwhile, the community itself continues to thrive.