What is a core search? It is search on one of the five major search engines, including partner searches and cross-channel searches. Searches for mapping, local directory, and user-generated video sites that are not on the core domain of the five search engines are not included in the core search numbers.

Why is this significant?

If you look at core searches in March, then Google accounted for 10.0 billion searches, followed by Yahoo! (2.6 billion), Microsoft (1.8 billion), Ask (593 million) and AOL (380 million).

Get it? Got it? Good.

But if you look at “expanded searches,” then you get a very different picture of the search market.

What is an expanded search? It is a search on the top properties where search activity is observed. What are these top properties? Oh, YouTube, craigslist, eBay, Facebook.com, MapQuest, MySpace, and Amazon.

If you look at these top properties, Americans conducted 23.9 billion search queries in March 2010. That’s right. About 8.5 billion searches were conduct that month on properties that generally aren’t considered “search engines.”

And 3.7 billion of these expanded searches were conducted on YouTube, which is less than the 10.5 billion expanded searches conducted on Google, but more than the 2.7 billion expanded searches conducted on Yahoo!, or the 1.6 billion expanded searches conducted on Bing.

And the 664 million expanded searches conducted on craiglist, 659 million conducted on eBay, and 647 million conducted on Facebook.com are more than the 594 conducted on AOL.

So, definitions matter. Since YouTube, craigslist, eBay, Facebook.com, MapQuest, MySpace, and Amazon don’t crawl you website, who is in charge of digital asset optimization for these top properties?

Ask your SEO specialist or webmaster that question at your next marketing meeting. And if they tell you it’s not their job, then tell them that you’re thinking of taking some of their budget so that you can create a new position.

Let me know what happens next.