Sorry, the term 'search' here means 'Yahoo! or Bing or Google's search services'. e.g. If you use Google Search, you might see a couple of ads somewhere.
I'm not sure that a search engine is a medium-- the medium would be the internet. The sentence also makes 'properties' a little vague if there is intended to be a difference, because I presume that advertisers also budget for ads on various search engines, browsers, etc.