Each user on Facebook, Reddit and Twitter has a value based on what investors think they are or will be worth to advertisers (advertising revenue) less the running costs of the business.
http://www.forbes.com/sites/georgeanders/2013/11/07/a-twitter-user-is-worth-110-facebooks-98-linkedins-93/#3da863619c12Steemit has no revenue from advertisers/other to cover running costs and most importantly is paying out large bonuses to attract new users and large amounts for their content. (Which other social media platforms get for free.) So Steemit is just a bottomless pit of expenses and no revenue.
Synereo on the other hand for example does have a business model. A social media site, where the majority of advertising revenue goes to the users in return for consuming advertising content. So that has the potential to be a lucrative, sustainable business model and Facebook killer.
The minute speculators stop buying Steem it may collapse because there is no business model or revenue source only the hope speculators will value a site attracting users and not use common sense.