Reading between the lines, they're not avoiding revenue solely to get/keep more users - I think they're doing a last man standing, and waiting for all their competitors to run out of cash and go out of business. Then they bring on the adverts, because there's no competition by then, and nowhere to jump to.
Technologically, it is very easy to start a competitor to Facebook/Twitter (the scaling is the hard part, not the starting). So, there will always be competition until the biggest players can somehow raise the cost of entry by a few orders of magnitude.
Socially, I don't think it is that hard for people to migrate from Facebook/Twitter or something else. I think a lot of people think about competition in this area happening like a huge number of people stop using Facebook/Twitter and start using a competitor almost simultaneously. That is kind of what happened in the past, but only because the old social networks had serious problems. Now, significant shifts will still happen but they will be a lot more gradual. A lot of people are on multiple social networks already (e.g. Facebook, Twitter, GitHub, Mixi, Linked In, Flickr). Over time, each user's the proportion of page views on each site changes gradually but often significantly. Facebook/Twitter might remain the site (almost) everybody has in common, but I doubt it will continue to enjoy the huge share of users' attention it has now.
That scenario is not likely to happen because the same Facebook users are also members of other social networks. It is not as if you are Facebook user, you won't have an affair with Brightkite.
Ultimately Facebook will end up being two simple things: the biggest address book (maybe) and the biggest photo sharing site.
Everything else will go on to sites that specialize in one area. Users will go to Twitter to share links and update their status, they will use Gmail for messaging, I'll keep using HN for Discovery and not the FaceFeed etc...
Basically, for the subset of things where having the most people you know on the site is more important than the fact that the interface of some other site is better, facebook may win out.
I know people who have resisted joining any social network that are succumbing and joining facebook. These are people who are not going to be signing up for half a dozen accounts on different specialty sites anytime soon. If facebook does it, they'll likely use facebook for it. If I want to keep in touch with them, or invite them to an event, facebook is the easiest way to do so. This means I'll use facebook more for these things, even though (unlike some of my friends) I'm perfectly willing to sign up for all sorts of specialty sites.
