This afternoon Yahoo announced that it was buying US-based ad network BlueLithium for $300 million in cash. Founded in 2004, the company has 10 offices globally and claims more unique US visitors than Google, MSN, AOL and eBay. It also claims to be the fifth largest US ad network and the second largest in the UK.

It’s part of Yahoo’s push to create “the largest and most effective online ad network globally.” That network includes the recently acquired RightMedia Exchange. The company will apparently operate as a wholly owned subsidiary of Yahoo and will report into Hilary Schneider’s new organization.

Most of what BlueLithium does is behavioral or demographic targeting, which has recently seen a big push by Yahoo via its new SmartAds platform. This acquisition can be seen as partly defensive and partly offensive in an effort to retain brand advertisers that have more choices today and to build out a broad and efficient online ad network that offers huge reach (incl. beyond Yahoo), as well as targeting and search.

Not long ago AOL acquired BT ad network Tacoda.

Related Topics: Yahoo: Business Issues | Yahoo: Display Ads | Yahoo: Outside US | Yahoo: SmartAds

About The Author: Greg Sterling is a Contributing Editor at Search Engine Land. He writes a personal blog Screenwerk, about SoLoMo issues and connecting the dots between online and offline. He also posts at Internet2Go, which is focused on the mobile Internet. Follow him @gsterling. See more articles by Greg Sterling