Even though Facebook is largely defined by the power users and the company's goal is to just keep ratcheting up engagement, I don't think that's the right way to judge it as a service.

As a casual user who visits Facebook a couple times a week, I find it to be tremendously valuable. Contrast with email for a moment. I somehow made it into my inlaws' family list. This is a group of perhaps 100 families who all manage their own copy of the list via Reply All. Many of them post multi-page updates of their domestic tasks several times a week. I can never be removed from or block this list because I may receive legitimate email from any of these people. I may occasionally actually be interested in what they are doing, but as it is the problem is worse than spam by a long shot. Now that some of them are getting on Facebook it is a huge improvement. I can block the truly annoying people and still keep ambient awareness of what's going on on my own schedule.

I think Facebook is a tremendously useful tool, which is often obscured inside the echo-chamber by the feeding frenzies around virality and engagement. In other words, I believe Facebook is sustainable.

Counter-anecdote: When I first moved here, I was very involved in the local music scene. In the last two years, I've pretty much lost touch with it completely; I noticed the other day that the curve tracked my gradual abandonment of myspace. Without those bulletins, it's hard to know when bands and venues too small for flyers are doing things.