IIRC, that was supposed to be our lesson from the dot.com bubble :) The early hype about "Web 2.0" was that this time around we're doing it for cheap and with a revenue model...
The "Web 2.0" mindset, at least the mindset I've seen most often on HN, is build it fast and get it out there to see if it has value. I also think there is some thought to how the solution can be monetized, but plans change in response to changing environments.
Current technology does make this valid in some sense - you can launch very cheaply and, if you attract enough interest, get bought by a larger company. As I understand this is YC's general approach. But it seems that Twitter tends to refuse buyout offers, so I they are more of a classic dot.com.
Sadly, this is not the Web 2.0 mindset. If it was things like Twitter and Facebook would have been "monetized" a few months after their launch, when it was abundantly clear that they offered value to people.