Google and Facebook are competing for ad dollars, but different ad dollars. Google has search advertising pretty much locked. Facebook has figured out brand marketing. Google has been trying to crack that nut for years with AdSense, but Facebook's built-in demographics and time on site looks like they figured it out.
I think commenter's are right. Time on site won't hurt Google's core ad market.
I think commenter's are right. Time on site won't hurt Google's core ad market.