A Mashup is according to wikipedia a web app that combines several data sources into one experience. Thinking about it that is exactly what millions of users do everyday on Facebook using the Facebook applications. Basically every application is a widget that is running in the Mashup Container Facebook. Each widget can interact with the world around it. So far there is no direct application-to-application communication that I am aware of, it is all via the Facebook infrastructure. But still each application can have access to your personal data as well as some data about your friends. This probably makes Facebook (one of) the worlds most widely used Mashup Container in existance today.
iGoogle and other widget platforms are not Mashup Containers in my definition until they allow the widgets to interact with the world around it. A widget that just sits there and does it’s little thing (like showing me the weather in my city) without being able to commuicate with the platform or other widgets is much less usefull than a widget that is part of a whole ecosystem. With OpenSocial Google, MySpace, LinkedIn etc are quickly becoming Mashup Containers in the same way as Facebook.
This is all very interesting because it means that millions of people are using Mashups every day, even building Mashups every day, all by using Facebook applications. How many of those people even know what a Mashups is? That means that Mashups are all among us already, but they are still undercover, much like aliens in some low budget sci-fi movies from the 50’s (“Plan 9 from Outer Space” rocks!).