The invitations come from individuals.
LinkedIn is like Facebook without the fun. It's 'potentially useful but dull, dull, dull. Google+ is potentially excellent and could displace Facebook and LinkedIn, but Google have still not quite worked out a good case for why people would use it rather than Facebook. Just not being Facebook is not good enough, especially if you try to impose unethical rules like having to use your real name on users... and Facebook thrives despite the evil intentions of its owners because it is fun and it works, and while Mark Zuckerberg may have no ethics at all, he knows how to give people what they want.