Say that it was your birthday three days ago. Your ex who you don't really talk to that much anymore but are friendly with fails to wish you a happy birthday even via facebook (which is probably the most casual way possible). Then three days later, he messages you to tell you that he's moving to Korea in a month to teach English. Still no mention of a quick "happy birthday."
Would you have not responded at all? Would you have killed him with kindness? Would you have said something like, "btw thanks for the bday wishes!"