We all know what Skillshare does. Here's a viral map...
Who are they? People who give lessons
Selfish reasons to contact friends? Because they want to increase their number of students and thereby their exposure/prominence on site, money earned, etc.
Monetization options? An annual or per-class rate user fee, a percentage of each class earnings, fees for placement in Skillshare's marketing material (regular newsletters or directory placement)
Motivations? Status (instructors want to rank as an expert in their field), Money (instructors want to gain SEO/exposure and referrals for their sites/businesses)
Who are they? People who take classes
Selfish reasons to contact friends? They earn referral fees, have more fun when a friend takes the same class, show it off to their social networks
Monetization options? Per-class fee, monthly or annual membership fee, additional fees for levels of access to instructors, fees for access to course material archives after the course is complete
Motivations? Status (students want to rank highly as top students or knowledgeable students, add it to their LinkedIn profile, show off to a boss or co-worker, "get certified" in a subject area), Money (students want to earn more via increased knowledge or certifications)