It's misleading to focus only on the direct benefits. The benefits of PR are always mostly indirect.
For example, Y Combinator makes zero direct revenue from hosting HN. But I would feel pretty dishonest if I were to claim on that account that it has no effect on our bottom line.
Exactly , his post is not objective. Front page of hackers news is essentially word of mouth marketing, which the author(Noah) says matters. Based on the data provided Noah does not know if the HN readers who signed up for the free accounts are recommending the paid services to others. So the value is at the very least $300 a month, it could be significantly more than that. Moreover what effect does the HN traffic have on their advertising, their job board, book sales ? He does not measure, so I doubt the accuracy of his conclusion.
I'm trying to quantify some of the second-order effects with a poll here: http://news.ycombinator.com/item?id=3680400 (FWIW, as I write this, the number of 37signals customers claimed by the respondents to the poll already exceeds the number of signups the SvN post states came from HN).
Front page of hackers news is essentially word of mouth marketing, which the author(Noah) says matters.
Not every kind of word of mouth matters. HN is not that big of community, and is comprised mostly of certain types of users that either don't care about 37's products, or know about them already anyway.
Based on the data provided Noah does not know if the HN readers who signed up for the free accounts are recommending the paid services to others.
Why, can't he check for affiliate links?
So the value is at the very least $300 a month, it could be significantly more than that.
I think you've misunderstood what PG was saying. In this analogy, Y Combinator and Basecamp are the products for sale and HN is a promotional channel. PG's point is that Y Combinator benefits from Hacker News in ways other than the direct "I clicked on the link from Hacker News and then immediately took an action that made money for Y Combinator."
For example, Y Combinator makes zero direct revenue from hosting HN. But I would feel pretty dishonest if I were to claim on that account that it has no effect on our bottom line.