Google released a public, updated monthly, list of the 1,000 most visited sites. The list does not contain adult sites or Google sites, but it includes mostly all the other sites that have AdSense ads on them, including Facebook which is number one on that list.
Do you think as an advertiser you would select only the top 1,000 or do you want any site Google deems relevant, based on the content of the web site, to show your ad? I can see pros and cons to both.
To only show your ads on the top 1,000 sites select 'non Ad Planner 1000' within the 'category filters' section of your AdWords account.
Forum discussion at WebmasterWorld.