Within the last few weeks I've been noticing that most of WP's ads that I get are from the "Ad Council". If you're in the UK or Canada, you won't get these. In the USA the Ad Council is a govt agency that acts as a clearinghouse for public service ads. There are laws that say that all media has to run a certain number of public service announcements in their ad stream. As new forms of media get invented, the courts have ruled that they are covered by the old laws. Since the Ad Council basically gets to run ads for free, American radio and TV stations typically run them during the traditional cheapest times, usually in the wee hours of the morning. If paid ads start drying up, the broadcasters will use Ad Council material to fill slightly more lucrative times. The same is true with the internet. When the recession bottomed out in 2010, Ad Council ads were everywhere on WP. As things looked up slightly, the Ad Council ads dialed back. Now they're back again-most every ad is one of them. Netflix, the last big ad producer, seems to have cut back, and nobody's filling their shoes, which means public service ads get shoved into the spots. This may mean that the economy is going downhill again.
Here is the wiki page on the American Ad Council:
http://en.wikipedia.org/wiki/Ad_Council
According to this their origins date back to 1941 and World War 2. That means that the public service ad laws are about that old. There is also apparently a similar organization in Japan, also called "Ad Council".