I think I am right in saying that Reddit did it by the founders posting lots of content as fake users. They even had a quick sign up form that their staff could

I think I am right in saying that Reddit did it by the founders posting lots of content as fake users. They even had a quick sign up form that their staff could use to create a new user and post. I think by creating the illusion of a very active community it made it easier to attract both search engines and new users. I am not going to comment on the morality of this. This has been discussed on HN before if you want to search.