The inclusion of settings for Facebook and Twitter don't change the fact that the OP's use case is atypical. Their data probably showed that custom ringtones for contacts are what most people need/want so that's what they built out.
Atypical? This is something I've wanted for years. Its just the ability to "Set Ringtone for all Contacts to..." its not that hard an implementation. OR alternately, a special ringtone for "unknown callers" or "Not in Contacts." Right now every call gets a default ringtone, its stored for All Callers. If its a call from someone in your contacts with a special ringtone, that gets played. NOT THAT DIFFERENT to store one where Anybody in your contacts gets one and anyone not gets another.
I would LOVE this feature. I have a couple of hundred business contacts and countless useless calls and a few special family and friends. Family & friends have a special ringtone, but useless strangers and my real contacts have to share, unless I manually change every single one.