Facebook users are far more likely to click on ads for entertainment, media sites, and blogs than for other kinds of ads.

This chart from a recent Webtrends survey of more than 11,000 Facebook advertiseements measures clickthrough rates (red) and cost per click (blue) for different categories of ads. There's a huge jump in clickthrough rates for the last two categories -- Media & Entertainment and Tabloids & Blogs. In other words, stuff that's fun to discuss with friends.

Health care ads are the least successful, followed -- somewhat surprisingly -- by ads for Internet and software products.