Facebook reached critical mass and now the majority of the people who keep them going are the same majority that don't see what all the fuss about HD is, don't want to buy a computer more than once per decade, are still satisfied with WoW, and think the current console generation is just fine as it is for another five or ten years.
In other words, they are people with minimal demands, needs, and expectations. They're happy with what they have now and aren't likely to move to the next big thing, like the rest of us always have.
Diaspora always had a significant uphill fight. Maybe they can use the project to bootstrap another effort in another arena at some point, but I think this is all a big effort to stand on principal. Not that there's anything wrong with that. Principal is important. It's just that principal isn't going to displace the reigning king. We will never ever see Diaspora Connect fueling the "likes" and "forums" of the web, everywhere you turn. And, unlike other services, social networking is one that really does necessitate a critical mass.