Well, one of them is Brett who is active here as well as has six planets, still didn't turn up so far. Since i can't invite him anymore i speculate that the invite went through (at least Brett said he saw it and took it). Still no icon in my bar. As far as i heard there is no max numbers of friends allowed for this game..
Edit: Rechecked the Facebook page and seemingly something isn't right there since there i can send him a friend invite (again) *cofused*