Another way to pose this question might be: among the successful YC B2B companies, how did they acquire their first few customers? I am sure that personal networks play a role, but then what fraction of initial B2B sales did they acquire from outside of personal networks?

This is the hardest problem, I think. Some "out-of-network" routes to market I can think of are cold calling, cold emailing, cold snail mail (FedEx, USPS, etc), cold LinkedIn "inmailing," participating in conferences (getting speaker spots and/or setting up booth), blogging, "ShowHN" style posts on online forums, advertising on search engines, advertising in industry publications, getting referrals from your investors (if you have investors) ... am I missing any? For every one one of these "hacks," you can find people saying that it doesn't work, that it's waste of time/money, that you should spend resourcing on building product instead, etc. Have YC companies succeeded in the earliest stage through these kinds of efforts? Are some more effective than others, for particular markets?

If it really is all just about personal network in the earliest stages, then that's equally puzzling to me because the numbers seem so low - how many prospective buyers could anyone possibly have in their personal network? Isn't it supposed to be a numbers game ("dial and smile")? It would take just a day or two to contact everyone you know who might be a buyer, what do you do after that?

The investor referrals idea seems like a plausible source of B2B startup success (especially if you are selling to other startups), since investors tend to be highly connected. I've heard YC partners say they can be very helpful in sourcing of your early customers. But another philosophy is that the only way to get into YC is to not need YC ... In other words, this leads to a chicken & egg problem because typically you need to get sales off the ground before any investor (often including YC) will invest in your company. It's the same as saying that you should get sales leads through customer referrals - that might not be super useful advice if you're pounding pavement to find your first handful of customers.