from http://bitcoinist.net/social-networks-decentralization-synereo-case/ :

"Synereo vision is to improve the model already presented by other social networks, but instead of being managed and regulated by a central authority, it uses blockchain technology to ensure a decentralized level of security and privacy fit for the next generation of social networking. Having this as the main approach, Synereo may well be the Facebook killer."

if invitation protocols were implemented it would be closer to Facebook/Medium. However, in the current iteration, it is more like Reddit. It definitely could replace Reddit or Medium, as those seem to be the main inspiration from what I can deduce. For me personally, since I use pretty much all existing social networks, Steemit has replaced Medium, Reddit and Quora for me. I occasionally look up stuff on Reddit still, but I hardly ever visit Medium or Quora anymore. This is a remarkable feat since I was literally hooked on Medium for some time. I used to read articles on Medium like 3 or 4 hours per day on average. I have not posted on Medium since I first found Steemit on June 8. That's significant.