Strange that you say that UI standardization isn't an issue on the web. What about the Jakob Nielsen "users spend more time on other people's sites than yours" school of usability design?
Actually, the world of Facebook apps. and widgets is the first time I've started to see that an old-style platform strategy may be possible. Here the basis is something which which is a hybrid of technology, namespace and social convention. Of which Facebook's "news-feed" is the archetypal example. Facebook's news-feed is not merely technological : which is why other generic data-sharing feeds like RSS or Twitter aren't equivalent. It's also a social convention within a particular namespace and community: I'm willing to look at data that an application writes on my friend's feed, even though I haven't installed the application or explicitly subscribed to it. This is different from the open web - I wouldn't welcome an ordinary web-application that my friend used, randomly spamming my email. (Similarly, if too many bots started writing to Twitter, that would kill that particular community pretty damned quickly, it's not part of its culture either.)
Facebook's platform power ultimately rests on their ownership of this complex but delicate socio-technical hybrid. If they can nurture and grow it, such as giving both users and applications, more and subtler ways to manage it, more nuanced types of relationships between people, with more fine grained privacy control and applications that access these both through the APIs and patterns of software behaviour, then I think they have something that's very hard to escape from or reproduce elsewhere.
This is no longer about just data, or arguments about open access to it. It's data + social data + social conventions.
October 02, 2007
Further clarification : here's a comment I tried to leave on Joshua's post (mentioned below)