It was posted to both Digg and Reddit on the same day. The Digg post never gained any traction and when the server went down under the load MrGrimm deleted the reddit post (but not the digg post, since it gained no traction).
Citation? My recollection is that imgur didn't show up until after the Digg exodus and happened on Reddit first, but maybe I just didn't see the Digg side of it.
He made a second Reddit post the next day.
Source: https://www.reddit.com/r/IAmA/comments/9tlwi/im_the_imgur_gu...