Google+ had an exclusive club appeal at launch because it wasn't instantly globally accessible, but slowly opened up instead.
It became clear they where desperate about user numbers when thay forced the merge of Youtube accounts. Or something like that.
Facebook was the same way when it started.
It was a little different. Facebook was eventually (after harvard only) wide open for college email holders so it wasn't some exclusive club that kept people you want to be in their out. It kept your parents out though and your lame younger sibling. You could immediately use it with your friends. No invite nonsense like with g+.