Oops, you responded pretty quickly... I wasn't satisfied with my answer yet:-) In any case:
Branding definitely is not network effects, though. Branding is what your earlier examples are about, like Apple. Network effects can be present even if you're not really that excited about the company - like eBay or Paypal. Actually, in some cases, where there's some exclusivity in the brand, they may actually work in opposite directions. Network effects push you to 'go with the crowd', whereas, say, Apple says to 'think different' and get a computer that not everyone else has.
I think expedia is a good example in terms of advertising/marketing, where people use it because they get out there and sell it. So perhaps that's part of the answer to keeping a DHH style company out front - sell it lots and sell it well. That's not something 37signals has ever shied away from. However, something like expedia is also probably not likely to ever be completely commoditized/open sourced, so there's always going to be some scarcity there.
That;s the brand right there. At least my definition of it.
> people use because they know it (which is one kind of branding), or because they want to be seen using it, as with your previous examples?
Point is, they're using it and not phoning a travel agent. They're getting something from it, that much is given.