On the Rhizome.org thread discussing "social media art," Duncan Alexander has a comment that's worth a read (except for the cartoon which reduces a certain blogger's keen insights to dog barking--am not particularly wild about that). Alexander says he's opposed to Alfred Barr-like timelines for social media art: agree, especially when they go back to the 1950s. He also resists start dates. Fine, you don't have to have a day and date some "great man" invented something. But you can peg roughly when the music changed. That would be the turn of the millennium, when "blogs" turned fixed web pages into dynamic pages and incorporated chat room chat as a kind of archival record. Since then you have had a process of converting blogs into vast networks of blogs, beginning particularly with MySpace. The art that we are talking about--such that exists--occurs within this continuum. Fixed pages still exist, but you likely will find them through the socialmediasphere and have them "spun" for you within that sphere.
Update: Alexander has already received his first dismissive comment (a picture of a teenager talking on a cell posted by "duh mom"). You gotta love the Socratic dialogue over there at Rhizome.