Smart aggregation would require some degree of collaboration and standards, and anyone supporting it risks making their own service a subordinate source.
Even though this kind of thing would be extremely useful to users, companies will only embrace it if the added value helps driving users to their own service more than to others.
Besides, if they did somehow integrate further you'd all be crowing about how Facebook was doing an embrace / extend / extinguish maneuver.
Imagine a solution that was open and allowed anybody to utilize its social graph. In theory this would allow other sites to build off the original and feed it back, maintaining equilibrium between every service. But in practice, this only holds until somebody decides to make a proprietary graph, because then users have nothing to gain by using any service other than the proprietary top level with the extra functionality.
And why would we do something like that other than greed? Well, perhaps we want to implement some unique system that's not streamlined or practical, but which we need for a particular plan we have. So we create a system that contains that extra, proprietary data, which is proprietary because making it open would simply make every other service messy. Or perhaps it's that what we want to do would take too long to implement in an open solution, and we'd rather be advanced than be consistent. But the more we build on, the harder and harder it is to move back to that original openness.
Similar to an environment like Flash, for instance, which lets developers implement functionality that's vastly beyond HTML (or, was vastly beyond, and still to some degree is), but which then ruins engines that rely on HTML's open format to work. Either you develop to be open, and sacrifice potential features, or you use something proprietary and lose that connection, and further place yourself in the hands of a third-party developer.
There isn't a single optimal solution, unless you consider using multiple platforms an optimal solution. (Which I do. I don't feel frustrated at using four sites to monitor my various friends/feeds/updates. I just be careful to delineate all four so that each item appears in the right place, and so that my own updates get pushed to the right places.
If we were selling cars, then preventing competitors from selling cars would likely mean that you sell more cars. However, if I view an article on reddit then I'm not really taking anything away from digg (except probably a view of that exact article).
If you do web design work in an Enterprise, you pretty quickly run into people who don't understand that there are fundamental differences between web apps and desktop apps.
What Web 2.0 does is it lets us get around some of those limitations. When Web 2.0 is done right, the Ajax is going to mean that they can work on an 'online app' without loading pages all the time (or rather, without appearing to load pages all the time).
Combine that with some of the database in a browser capabilities, and, in theory, you could get around even more of the limitations. E.g. user closes the browser window before hitting submit means the data is lost = annoyed users = poor usability. But if you use the offline storage to save the fields then it isn't lost after all. Hooray!
But yes, Facebook is retarded. Build a better one I dare you. ... Please.
Interestingly, in the German, he used a completely different example:
"What the devil is the whole web 2.0 crowd doing all day? I mean, other than sitting around in St. Oberholz , sucking down Bionade  and Twittering their brains out?"
 Berlin equivalent of Red Rock Coffee in Mountain View
 Trendy organic fruit soda
The easy technological solution is a site that offers complex, configurable relationships, but can people handle it? Probably not, or at least, not yet. So we have reached the current equilibrium of multiple services, with Facebook the winner, and a minority of people choosing integration or choosing to manage accounts separately on more than one service.
Perhaps the day will arrive when people are ready for more sophistication in managing relationships on a social networking site. If Facebook times it right and adds relationship configuration options at roughly the right pace, they can stay on top and supplant Twitter entirely for social networking. Or perhaps the current situation reflects the limit of what people want to cope with. I'm sure Facebook is researching the question and deciding which way to bet.
"Yes, it is possible. Each of these services has an API, and I have to authorize them against each other anyway. So could these services use their APIs and offer useful filtering preferences to me, pretty please?"
I think this, along with the ability to show what pieces of information are most important, would be a killer features. The network that could implement this would do a lot of the elimination of noise that plagues using multiple social networks.
The problem with using our friends' activities as predictors of interest is that Facebook has no way to distinguish between strong and weak friendships. There is a marked difference in my life between Facebook friends and meatspace friends.
 - Sure, they can determine which of my friends I spend the most time talking with on Facebook. But that says more about our relative Facebook activity than the quality of our friendship.
The real issue is that cases like those of anthuswilliams above you are unique and difficult to replicate. That's one of the noted problems with music recommendation services like Pandora too: What matters isn't what users tend towards but why, and why is a question that can't be effectively computed. It's more freeform.
I think the real solution would be a site that actively helps a user be responsible for himself. Something that doesn't simply indicate what features are available, but why users might want to use them to sort out their friends. Contrary to what the OP says in his blog post, that is possible today, but it requires responsibility on both the parts of the reader (who must sort his inputs neatly) and the publisher (who must figure out where to publish what). And web sites presently don't feel it's worth their time to help you with that task.
Now, interesting is different than "like", which is different than passion or love, which are all different than studying for school or work, or an obligation.
But anyway, a very good question, you gave me inspiration to delve on this topic and come up with lots of different approaches to determine what people will thing it's interesting.
Furthermore, what algorithm would be best? A type of Bayesian filtering is really effective at separating spam from non-spam, would it be effective at filtering interesting from non-interesting?
Applied to a social graph, it would (hypothetically) find content that is both recommended by friends and probably new to you.
I am a native german speaker.
Which says a lot about Google Translate, or my reading comphrehension; I am not sure which one.
Facebook, on the other hand, doesn't really seem to be doing much with its data. But it does have an extremely powerful API, so, maybe someone could try taking this matter into their own hands.
In the real life, friends rarely go through the formal notion of "accepting the other as a friend" (aka send/accept friend requests). Friends are those who I talk to gladly and often. Simple as that.
I've found that the less technical and "webby" people are, the more they understand Web 3.0
A lot of that is that, in Web 3.0, perhaps 70% of what's necessary for Web 2.0 is superfluous... Perhaps nice to have, but frankly, the next generation systems need a community the same way that "The Terminator" needs people.