In what sense do are those sites "service businesses"?
The whole point of the service vs. product distinction that PG was making is that a service business is less scalable: its revenue is basically linear in the number of employees that it has, because a human being has to do more work for each new customer. That is not the case for product businesses or for Reddit or Octopart (or most other web startups) -- if Reddit's traffic increases 100x overnight, they don't need 100x the employees to capitalize. Craigslist is an example that makes the difference pretty clear. Whether you want to call them "product businesses" or not misses the point.
It's all a question of where you draw the line, I guess. There are consultancies, which scale 1.5x. There are service providers, which scale 2-3x. There are ASPs, which scale 5x. And there are product companies, which scale 10x.
Reddit is still different from an actual product company. The Reddit product scales independantly of the number of redditors or subreddits, because the Reddit product isn't hosted.
