Ethereum would be crushed under the kind of transaction volume Steem is handling right now. That's because Ethereum hasn't even begun to transition into Proof-of-Stake. An Ethereum-based social media would have a snowballs chance in hell to gain any traction. The only viable competition to Steem might be based on EOS. There is a lot of development taking place on Steem right now and that's the most relevant metric when assessing future potential.