Source: "If you’re going to launch a startup, how many friends do you need?": http://buzzpal.wordpress.com/2008/01/06/if-youre-going-to-la...
Nor was Amazon, really: Bezos had a de facto cofounder called Shel Kaphan, who wrote all the initial code.
"Over blueberry pancakes at the Sash Mill Cafe in Santa Cruz, Bezos managed to convince one of them, Shel Kaphan, to become employee number one."
A bunch of the other ones seem suspect too. Wikipedia lists the founder and first CEO of Lycos as Bob Davis - Michael Mauldin invented it as a research project, but if you go by research project inception and not incorporation, Larry Page is the sole founder of Google.
Be wary of information from random blogs. ;-)
Berkshire Hathaway is a textile mill that Buffett turned into an investment company, but by then he had been a successful investor for over 10 years.
It's hard to lump financials in with tech startups, though. Technically, when he Buffett started the Buffett partnership, he had lots of cofounders. The structure of an investment fund is a single managing partner (who makes the decisions) and then lots of limited partners (who put up the money). In the eyes of the law, they're all cofounders.
If you go by practical instead of legal definitions - who provided the know-how? - then Buffett's early partner was effectively Benjamin Graham. After all, it was at Benjamin Graham's firm that Buffett first learned about investing, and Graham served as a mentor long afterwards.