The top 2 (Amazon & Ebay) where founded in almost a different internet age. It makes wonder if a solo founder could create such a large company today.
My night project, PaperBox (gopaperbox.com), started with just two guys doing it a combined 20 hours per week or so. Over the course of 6 months or so, we have released several updates and grown substantially. I do wonder what we could be if we could commit to it full time and have any sort of a marketing budget.
That said, the way we use the internet today is changing which is why I don't think its exactly impossible but would question how difficult it would be in comparison to 20years ago.