From what I see, Reddit is terribly undervalued. It's chasing advertising dollars in a game that it cannot win (FB has much larger scale and ads inventory to sell). Take a look at this before thinking of investing in the IPO--> https://www.readthehypothesis.com/p/reddit-valuation-monetiz...
Google's recent research throws light on how using psychology principles such as Social Proof, Power of Free, etc. can be used to build super-charged brands/products.
This short ebook looks at 4 psychological principles that are used by apps such as Strava, Canva, Duolingo, Trello and many others to build user-centric products. There are 35+ real world examples as well apart from theory.