A piece of software can be as great as it can be but if there is no one to check out then who will use it and talk about it?
Before launching Stackoverflow, Joel had around million pages views per month on his discussion forum and blog. I don't know the exact figure but it was definitely a huge and combined with Jeff's blog traffic it is massive traffic.
So obviously if you show your visitors something new and shiny they are definitely going to check out and if the product is real good then the users are going to be falling in love and spreading it all over the web. That is what happened in my opinion. This just seems all automatic but there is lot of effort spent on marketing and timing the actions.
Every success requires hardwork and lot of effort. And every effort and hardwork needs marketing in one way or the other. To the outsider it always seems like an overnight success but it is not.
Do the copycats have that kind of visitor volume? I guess not