four levels in the hierarchy of Web 2.0-ness. Level-3 applications, the most "Web 2.0"-oriented, only exist 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 as examples
eBay,
Craigslist,
Wikipedia,
del.icio.us,
Skype,
dodgeball and
AdSense. 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 can rank as "level 2"). Non-web applications like
email,
instant-messaging clients and the
telephone fall outside the above hierarchy