Sorry to chime in uninvited. I think, you can not compare only coinbase either. There are plenty of applications running on bitcoin right now, starting from bitcoin banks like coinbase to faucet sites. Steemit wont come anywhere close to the combined value of those sites. But, that does not matter.

Bitcoin has got 7 years, whereas Steem has hardly got around 7 week so far. Moreover, these two serve as complimentary in my opinion. I wrote an article regarding this. May be you can check - Bitcoin and Steem together is a Deadly Combination.