I’m just wondering what kind of reward(s) you’ll get if you invite friends (and get them to level 10) at the number of 6 to 9 and 11 through 19 (?). There’s only rewards for 1 to 5, and then only 10 and 20… 
I know this isn’t a big issue for most of us, since it’s really difficult to recruit friends through the game interface; most people (even your closest friends and family) think of your message(s) as “infomercials” or even just spam, so if you don’t guide them step by step manually, you will will have a success rate close to zero - even my mum (RIP); who loves these kind of games, didn’t bother to respond. And when I asked her why not, she said she thought it was just spam and deleted it right away. And when I tried to guide her through it, she meant it was to much of a trouble, and aborted half way through.
But even so, this has been bugging me for a while now, and I really want to know what happens when you recruit the number of people I mentioned in my introductory question.
Anyone?