Co-founding is often spoken of as if it's a binary state as Mark points out, it's not. A cofounder who's not 50% committed is a more expensive and less use than a 100% committed first employee. First employees take salary though which takes investment and investors require co-founders.

It's a shame that the data on this is going to become more and more sparse over the next few years as investors locks in on multiple co-founders and single founders become more and more rare.