Extensions show your status and your respect towards someone.
Calling someone by their first name or family name depends on your friendship and status too.
Sakura and Syaoran used to call each other "Li-kun and Kinomoto-san"; they then became friends and called eachother "Syaoran-Kun and Sakura-chan".
-Kun = friends that are boys (Li-kun)
-Chan = endearment/friendship (Tomoyo-chan or Kero-chan)
-San = respect (Yukito-san)
-Sensei = to some one who is teaching you (Terada-sensei)
-Sama = to someone you have great respect for (Chaos-sama from TRC)
-Hime = Prince or Princess (Sakura-hime from TRC)