Facebook is primarily about social interaction - sharing photos from last night's party, listing what music you like etc.

Academia.edu has an exclusively academic focus; it's about creating an academic webpage, uploading your own papers and research interests, and finding others around the world who have the same research interests as you.

Although most/all academics are aware of facebook. If they had an interest in this they would be using facebook to do it.

You mean: Isn't Facebook a "Friendster/MySpace style site for academics?"