Click Here: The Short, Inglorious History of Online Advertising

1994

Pay-per-click keyword advertising debuts at GoTo.com (later Overture, and now a part of Yahoo). It is widely mocked.

1998

HotWired is the first site to sell banner ads in large quantities to corporate advertisers. The first buyers? AT&T and Zima. Click-through rates, the gauge of success, are an astonishing 30%.

2000

Google rolls out AdWords, a pay-per-click service. AdWords made performance-based ads mainstream and now accounts for more than 95% of Google's revenue ($21 billion in 2008).

2001

Pop-up (and pop-under) ads fill users' screens. They peak in 2003, at 8.7% of all online ads. Although initially effective, earning 13 times more clicks than banners, pop-up blockers end the annoyance almost as quickly as it began.

2005

Video ads bring in $121 million -- just over 1% of online ad revenue. Today, they're still a small slice of the overall market as YouTube, Hulu, and others experiment with formulas, but video-ad revenue jumped to $500 million in 2008.

2007

Facebook debuts its advertising system, including Beacon, which raised members' hackles for being an opt-in program by default. A month later, CEO Mark Zuckerberg allows users to turn off Beacon -- and they do just that.

2009

Companies such as Lotame begin to experiment with engagement as the new ad unit on social networks. The goal? Attract more than the $2.35 billion that companies are expected to spend on social-network advertising this year.