Up + Power?! That's the screen capture combo keys... Did you mean Down + Power?
... go to Settings - Lock screen and change notifications - choose an app to show detailed status to Skype. This will allow Skype to bleed through the lock screen and then you can unlock and answer the call. ...
I don't have Facebook installed...
I forced to uninstall Skype.. otherwise every single call (incoming) freezes the phone..