Below I have reproduced the figure first made by Jim Hamilton and Richard Green and then later popularized by Mark Thoma. This figure suggests the following: (1) Fannie and Freddie (the GSEs) gained market share beginning in the 1980s from the saving institutions (presumably from the Saving & Loan debacle fall out); (2) Fannie and Freddie lost market share beginning around 2002 to the asset-backed security issuers. As noted by the above observers, this latter point supports the notion that at least some of the problems at Fannie and Freddie emerged in response to their declining market share during the housing boom. In other words, what happened to Fannie and Freddie may have been a symptom rather than a cause of the housing boom-bust cycle. (Click on figure to enlarge.)
Update: This article from the National Journal argues that Fannie and Freddie played a larger role than I outline above.