He breaks it down into 3 levels :
Level 1 platforms provide an API for external applications to call their services.
Level 2 platforms (eg. Facebook) help the external application present themselves by incorporating the applications within their UI. I'd say that we're really talking about platforms which provide a "callback" to the application.
Level 3 platforms host and execute the application themselves. With the benefit that the platform can broker richer integration between the applications. This is obviously what Salesforce's Force does. And, of course, Ning - the best part of this email is that it signal that Andreeson is trying to promote the "development environment" idea of Ning which was the exciting idea that kind of got lost when they rebranded it as "just another social network". (Asde : In fact, does Ning have what it takes to become a budget Force / AppExchange rival?)
He goes on to make a very good point :
Second, beware overfocusing on the apps of the past when thinking about the platforms of the future.
Lots of people got confused by the idea of apps running in the browser because when they thought of apps, they thought of the apps they used already on their PCs -- Word, Excel, Powerpoint -- and not the apps that would get built on the web -- eBay, Amazon, Salesforce.com. Now, it turns out in the fullness of time that word processing, spreadsheets, and presentation apps are also moving into the web -- as Google is demonstrating. But way before that happened, the web led people to create lots of new kinds of applications that were not possible on the PC.
A new platform typically enables a new set of applications that were not previously possible. Why else would there be a need for a new platform?