HIERARCHY OF WEB 2.0 SITES

* Level-3 applications, the most "Web 2.0"-oriented, exist only on the Internet, deriving their effectiveness from the inter-human connections and from the network effects that Web 2.0 makes possible and growing in effectiveness in proportion as people make more use of them. O'Reilly gave eBay, Craigslist, Wikipedia, del.icio.us, Skype, dodgeball, and AdSense as examples.

* Level-2 applications can operate offline but gain advantages from going online. O'Reilly cited Flickr, which benefits from its shared photo-database and from its community-generated tag database.

* Level-1 applications operate offline but gain features online. O'Reilly pointed to Writely (now Google Docs & Spreadsheets) and iTunes (because of its music-store portion).

* Level-0 applications work as well offline as online. O'Reilly gave the examples of MapQuest, Yahoo! Local, and Google Maps (mapping-applications using contributions from users to advantage could rank as "level 2").

Non-web applications like email, instant-messaging clients, and the telephone fall outside the above hierarchy