I figured it out. Here is the block of code that works for me. I welcome suggestions for improvement :)
self.myStore = [[ACAccountStore alloc] init];
ACAccountType *acct = [self.myStore accountTypeWithAccountTypeIdentifier:ACAccountTypeIdentifierFacebook];
NSArray *fbAccounts = [self.myStore accountsWithAccountType:acct];
// Check to make sure the user has a FB account setup, or bail:
if ([fbAccounts count] == 0) {
userDoesNotHaveFBAccountSetup = YES;
self.fbSwitch.enabled = NO;
[[self fbSwitch] setOn:NO];
[self viewDidLoad];
} else {
userDoesNotHaveFBAccountSetup = NO;
self.fbSwitch.enabled = YES;
}