I’ve wondered about this but not as thoughtfully as Ben Compaine, who posts on the Rebuilding Media blog:

Can the media survive on advertising? Lots of folks are counting on it. Broadcasters have always had this single revenue stream. Daily newspapers get about 80% of revenue from advertising and the hot print properties, such as the give-away Metro dailies, depend about 100% on advertising. Now much of the Web is counting on advertising: Google, Yahoo! and increasingly AOL to name just a few of the biggies.

Either the pie gets bigger or somebody gets a smaller slice.