The only thing I can immediately think of is that you might have some general setting to do with Apps disabled. If you don't have any Apps on your account, then there's a good chance this is the problem, and I think you should be motivated to try temporarily turning on Platform by turning on Apps on the first setting on this page.

On looking on Facebook, I see it has managed to add the Stellar app. But still, when I try to "connect with Facebook" it opens a popup window which immediately goes away, and it reports "Oops! Please verify your Facebook account and try again." Which I have done ("verifying" apparently means "tell Facebook your mobile number") but with no change. Well, maybe I'll contact Stellar about that.