According to ComScore, Facebook’s share of the display advertising market during 2010 Q4 eclipsed the 25% mark for the first time, eroding share mainly from Yahoo. Most likely, this has increased even more in Q1 2011.

Here’s the full article with the rest of the ComScore data:

http://www.mediapost.com/publications/?fa=Articles.showArticle&art_aid=149597&nid=126287