I believe Smart said in an interview that he talked to JB on the phone shortly after being traded.
These public commiserations / goodbyes / thank yous on social media do not matter.
gary washburn: Talked with Marcus Smart today in Stoughton: 1) He was told week before the trade he was safe 2) Found out through his agent, not team. 3) Said he’ll always love Boston & #Celtics organization 4) Adamant that he & Jaylen Brown have no issues. Talked with JB shortly after trade – via Twitter GwashburnGlobe
This makes perfect sense to me.
Most people I know, when a birthday comes along for somebody they know, goes and posts on that person's public facebook/instagram/whatever page with happy birthday messages, etc.
For me personally, if I'm close with somebody I would much rather send them a private message or make a phone call. It just feels much more meaningful and personal that way - it's like I'm contacting them to wish them well, not posting on their page for appearances.