Facebook and Google are examples of great ideas executed well creating huge markets, which did not exist when they set out. So how would have their founders fared if they chose to focus on a great market at the start? We would have yet another portal with banner advertising and a dating site.
I agree with the point of the post you mention here, but your post doesn't really support that.
Your advice is to focus on a great market. One of the hottest markets right now seems to be photo sharing. But I highly doubt the next great company will come out of there.
Can you give me an example of a great market today?