This is a dangerous statement. On the one hand, he is right in that it's best to focus on core values over monetization in the early stages so that you capture a maximum following, however if you change course too much at a later point by injecting too much advertising, then the amassed following will go elsewhere.
The excess number of people joining facebook will lower the quality of the network. The introduction of ads will make the site less pleasurable. But, other start-ups won't suffer from these problems, so my bet is that one of them will take over as the "new facebook" a couple of years down the road.