The point of an S-1 filing is that Facebook had to disclose certain information and was required to operate in a certain fashion. That's big regardless of how many shares were previously floating around.
Also, correct me if I'm wrong but I believe Facebook has only sold shares publicly on a fairly small portion of its huge valuation so in a sense you might argue that valuation hasn't tested that much.
FB's public float appears to be approx 5% of the company's total value. Though relatively small, the stock price of each share should reflect the company's total value. That being said FB is fairly unique in many ways and has been able to draw demand for its equity both as private and now a public company, that appears to defy typical valuations and funding scenarios. The writer maybe correct about FB's current access to capital as a private company, but secondary markets for most companies are generally not so liquid or transparent for most investors.
Also, correct me if I'm wrong but I believe Facebook has only sold shares publicly on a fairly small portion of its huge valuation so in a sense you might argue that valuation hasn't tested that much.