Given the Facebook community's tendency to be irritated with change (if only for a week or so), anything that effects the status quo could be a catalyst for Google+ adoption.
My guess is Music: <a href="http://www.insidefacebook.com/2011/06/20/music-dashboar/" rel="nofollow">http://www.insidefacebook.com/2011/06/20/music-dashboar/</a>