In the initial design of the web reading and writing (editing) were given equal consideration - yet for fifteen years the primary metaphor of the web has been pages and reading. The metaphors we used to circumscribe this possibility set were mostly drawn from books and architecture (pages, browser, sites etc.). Most of these metaphors were static and one way. The steam metaphor is fundamentally different. It's dynamic, it doesn't live very well within a page and still very much evolving.

A stream. A real time, flowing, dynamic stream of information -- that we as users and participants can dip in and out of and whether we participate in them or simply observe we are a part of this flow.

http://www.borthwick.com/weblog/2009/04/19/699/