Don't bother with the rest of the Ender Quartet. While they're not terrible, they don't hold a candle to Ender's Game. There's just better uses of your time (like any other book on this page).
If you have out-of-band connections with all your friends and you have friends who are willing to jump through weird, nerdy, technical hoops, what do you need Facebook for?
There would be no out-of-band connections. The extension uses Diffie Hellman or something to swap keys with your friends over Facebook (or maybe through a server run by the extension provider.)
There's another factor at play in Samantha's case - she was adopted. The article mentions that no evidence of abuse was ever uncovered by previous parents (both biological and foster), but who knows?
You're right, they did adopt a half sister who may have not shared any family home with Samantha at all or was cohabiting a home, could have used some more info on that.