Synereo is supposed to scale because of sharding and because anyone can host a node. It's not designed like a typical blockchain datastructure but instead is something else. The same person behind Casper (Ethereum's approach to scalability) is behind Synereo. The information isn't mentioned much in the whitepaper because the whitepaper is from 2015, but a lot of their discussions are on Youtube in the form of hangouts. I believe I posted some of the Casper discussions, and there are many many hangouts.

From what I recall they have a good design for scaling a social network, in some ways better than Graphene because it's more decentralized, because in theory it uses a proof of resource model where anyone can sell storage, or bandwidth, etc, but not enough hard data is known beyond theoretical. Sharding is a theoretical mechanism to achieve scalability, almost like load balancing, but until they produce actual statistics I will say it's unknown whether it will be better than Graphene. You would think that considering they can just look at Graphene and copy the best aspects of it, that they should at least be able to match Graphene, but you never know until the results come in.

References

1. https://en.wikipedia.org/wiki/Shard_(database_architecture)