I have wondered about that too Alex. Without having access to site analytics it is difficult to know for sure but I believe the large bulk are non-steemians who just come in through search engines.
It would be great to have more information on this. I guess Steemit Inc will be analysing all the data for their advertising sales now.
Ooh, organic none member traffic! There’s a thought, well steemit does Rank well on google.. id never have guessed it would be so high! Those steem ads should be paying dividends then! X