One of my favorite examples is YouTube. They actually were about as close to "overnight success" as it gets. It was still something like a year and a half before they had significant traffic and most of us heard of them.
That's not accurate -- they launched the domain in Feb 2005; they uploaded the first video April 2005; they officially launched in November 2005; then Lazy Sunday went viral in December 2005.
They were already well known before Lazy Sunday, but that tipped them over the edge.
YouTube the company was an overnight success, if anything can be called that. I wouldn't say the founders had overnight success, but pretty close to it. They put in some time at PayPal and probably made out quite well from its sale, which allowed them to start YouTube. Some people just have talent + luck.
