The most obvious difference is that I know who are the users of these sites. Google+ might be popular in a geographic part of the world, or among a certain com

The most obvious difference is that I know who are the users of these sites. 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.