The Wall Street Journal published this infographic featuring a comScore analysis of the breakdown of large and small advertisers for the top three advertising publishers in the U.S. (Facebook, Yahoo! Sites, and Microsoft Sites). In Q3 2011, Yahoo! and Microsoft shared a similar profile with smaller advertisers delivering around 21 to 23 percent of ad impressions on these properties. In contrast, 62 percent of ads delivered through Facebook came from smaller advertisers.
This chart shows that Facebook appeals greatly to small advertisers, likely due to its intuitive advertising interface minimizing barriers to entry. It is important to note, however, that Facebook is still competitive with Yahoo! and Microsoft when vying for ad dollars from large brand advertisers.