The date the Wall became part of Facebooks profiles was in summer or fall 2004 just a few months after Facebook’s launch the previous February.
It was originally formatted as a wiki, where any of a user’s friends could edit the entire contents of the Wall and also view a revision history. As I understand it, it was meant to replicate the whiteboard that you often found on dorm room doors, complete with the ability to erase everything previously written on it.
Over time, it was observed that people rarely erased earlier postings on a given user’s Wall and tended to post new content at the top. As a result, most people’s Walls were simply reverse-chronological lists of comments, but without directly visible attribution to who posted each comment due to the wiki format. Many Walls also became extremely long as old content built up over time.
The decision was made to change the Wall to its (roughly) current format of individual posts in August 2005 as part of the first major redesign of the site. (This redesign also included Facebook changing its name from thefacebook.com and adopting its current logo.) The change was initially met with much hostility from users (as were virtually all changes to the site in its early years). Many felt like it made Facebook profiles too closely resemble those on MySpace.
The initial backlash quickly subsided, and the Wall quickly became the focal point of profiles and user activity on Facebook. This led to later changes that essentially combined the Wall with the Mini-Feed of a user’s activity that was added to profiles in 2006. By late 2008, the Wall was, in effect, synonymous with a user’s profile.