Former Google (NSDQ: GOOG) and DoubleClick exec Michael Rubenstein is joining AppNexus, a company that provides online advertising via a cloud computing platform. Rubenstein has been named president of the New York-based company, AppNexus said in an e-mail release. He will report to AppNexus CEO and co-founder Brian O’Kelley, who was previously at Right Media before it was acquired by Yahoo (NSDQ: YHOO). Rubenstein was one of the key executives at DoubleClick and joined Google after the online ad company was acquired two years ago for $3.1 billion. As we noted earlier, Rubenstein left Google at time when many high profile execs have headed out the door. His departure also comes as Google is attempting to get its display ad business off the ground through DoubleClick’s Ad Exchange platform. A year ago, AppNexus raised $8 million in a second round funding led by Venrock. Other backers have included Kodiak Venture Partners and angel investor Ron Conway.