I want to put a decent entrepreneurial argument. We have seen many Y Combinator startups up till now and I think many had great potential and could have expanded their business and user base to match with the titans.
Why we didn't see next facebook or Digg beaters(in terms of size)? I think only Scribd, Loopt and Reddit have gone so far and they had much more potential to go further considering their business models ... or I am being a jerk? Of course I don't know the complete picture and it's too early to tell this, but I would like you to give a different dimension to this argument and tell us your opinion.
Huge companies with huge success are very uncommon - that's why we can list them off the top of our heads. YC invests in a lot of companies, but not many in terms of the total number of startups in the world. So lets say that out of 10,000 startups, 1 gets huge (Google, Facebook, etc), 9 get big (Digg-sized). That's 1 in 1000. YC has invested in ~80 startups (call it 100 for easy math sake), which is 1% of the "market". 100 tries at 1 in 10,000 isn't great. Even if you say that YC companies are 10x more likely to succeed (not unreasonable given the training, motivation, selection bias, etc), 100 shots at 1 in 1,000 still isn't that good.
Becoming huge requires a lot of things - including luck and timing. No matter how much value YC adds, it can't add those two things. What it can do is increase 1) the odds of success and 2) the expected value of that success (0.1% chance of $1B, 99.9% chance of $0 isn't very appealing).
I think pg's goal isn't to create billionaires (although he wouldn't mind). His goal is to help smart hackers solve "the money problem" so they can have the freedom to pursue other productive pursuits they wish. Look at what he, rtm, and tlb have done since they sold Viaweb (Arc, YC, HN, Harvard prof, awesome homemade robots). None of that would have been done if they were working at Google for $110K/yr.
Trying to solve "the money problem" in a couple years shows an excess of ambition, especially since programmers earn decent coin in the corporate world.