As a matter of fact, Steemit would still grow in numbers exponentially even if it chose not to advertise because of its Godsent built in advertising and marketting nature of Steemit.

What kind of advertising do Steemit do to get new dolphins and where? / 7-26-2016 / @kakradetome / https://steemit.com/steemit/@kakradetome/steemits-godsent-inbuilt-marketting-strategy-and-why-they-don-t-have-to-pay-a-dime-for-marketting / Steemit Advertising and Marketing

Nonetheless, Ned and Dan mentioned in their blocktalk interview, there could be a referral System.

Referral System / 7-25-2016 / @kakradetome / https://steemit.com/steemit/@kakradetome/steem-referral-program-coming-soon-and-what-you-should-do-while-it-s-not-here-yet / Referral System