Just before Facebook's much ballyhooed IPO a week ago, news got out that General Motors would be pulling its ad spend from the social network, causing quite a bit of discussion about Facebook's revenue potential and, more importantly, how digital advertising activities should fit into the marketing mix. Since that point, cooler heads appear to be prevailing. While there's no “right” answer for whether GM's move was the proper one for the brand just yet, at least we're spawning discussions ...