What is RChain?
There's actually no comparison, because just like how Ethereum was said to be the next level of the Internet..RChain (which is the technology that Synereo will be built on) is said to be the next level of blockchain technology (Blockchain 2.0).
It is based on Ethereum’s Proof of Stake protocol Casper which here is a recent blog by Vlad Zamfir says:
Casper “the friendly ghost” is an adaptation of some of the principles of the GHOST (Greedy Heaviest-Observed Sub-Tree) protocol for proof-of-work consensus to proof-of-stake.
He states that "Casper’s blockchain does allow for faster block times than is possible with proof-of-work consensus" and he goes on to mention..
Casper is a security-deposit based economic consensus protocol. This means that nodes, so called “bonded validators”, have to place a security deposit (an action we call “bonding”) in order to serve the consensus by producing blocks. The protocol’s direct control of these security deposits is the primary way in which Casper affects the incentives of validators. Specifically, if a validator produces anything that Casper considers “invalid”, their deposit are forfeited along with the privilege of participating in the consensus process. The use of security deposits addresses the “nothing at stake” problem; that behaving badly is not expensive. There is something at stake, and bonded validators who misbehave in an objectively verifiable manner will lose it.
Source: https://goo.gl/ltldEs
So in the future of blockchain style apps and social networks.. Synereo is taking a huge leap with Ethereum's RChain technology by allowing the site to be able to handle a large scale user base!
Even though Synereo is scheduled to launch later this month.. RChain is not expected to launch until Q4 2017.