Well i never get this message because i only add my real friends at fb , though my friend has told me about it as he adds friends randomly there , that could be your problem as well.
Maybe fb is getting smart in the aspect that people are going thru and seeding out friend invites to people that they don't even know... why would you invite someone to be your friend if you don't even know them? Way to go FB! I am sure that I would be more excited if I had an account with them.
I went to a group in the UK and attempted to add just 1 person, which then showed me that message. I think it's a new feature they're (Facebook) trying out to scare spammers.