Would you have made a similar comment if these numbers were for some other site, say Baidu or Rakuten?
Google+ might be popular in a geographic part of the world, or among a certain community I'm not part of, just like Baidu is in China, Rakuten is in Japan, Vkontakte is in Russia, Orkut in brazil, etc.
I just have no idea, and it seems I'm not the only one, what this demographic they have conquered is.