It's not only about adding friends too fast, it seems to also take lots of other things into account. Geography is one, if all of your friends are from Sydney, but you try to add someone from London with no friends in common, you may get questioned. People denying you also doesn't help.
My advice to anyone trying to add is to find a town, add a bunch of people and build up your mutual friends, you can easily get to 1000+ in a town with only 50k people.