(Image source: Getty images) Facebook Inc (NASDAQ:FB) is trading higher today as a result of a buy upgrade by Citigroup Inc (NYSE:C). The price target was raised to $55 a share from $32. The analyst on the upgrade call, Mark May cited “sustainable growth” in mobile ads as a justification for the upgrade. According to Bloomberg, 40 percent of advertising revenue in the second from Facebook was mobile related. At the current moment, Facebook is trading above 3 percent.