For reddit, all the content was submitted by Steve and Alexis at first, then they got a couple of friends to start submitting.

To make submitting for themselves easier, Steve made a special submit page that would also create an account at the same time, and then make the submission from that account.

After a while they noticed people they didn't know were submitting, and then PG mentioned them in an essay and things started taking off.

They knew they had something the first day they didn't have to submit anything.