Mark Zuckerberg's Google+ Profile, What is Facebook owner doing here at Google Plus

Mark Zuckerberg, The Facebook owner, the website which having millions of users in their database all around the world has recently re-open his profile at Google+. Nobody know, who give him invitation of Google+ and how Mark Zuckerberg get Google plus invitation. Google did not invite him for Google+. Here is screen shot of Mark Zuckerberg profile at Google+ and their is no new message to display at Mark Zuckerberg profile.