The Celtics social media strategy seems to be way off-based. "We almost never reply to our fans" seems like a terrible strategy for social media. It's called 'social' for a reason and just talking AT your fans and not WITH them seems like an old-school marketing attitude out of Mad Men.
Now obviously fans will be upset after a loss and it's not smart to respond to fans in that state of mind, but not involving fans in your brand through social media is a bad idea in my opinoin.
As for their use of Pinterest, they are not geting any followers on it because they are using the social network incorrectly. Their focus on just selling merchandise is not what the platform is made for and their numbers reflect that. They should take a look at the Seattle Seahawks and their 144,000+ followers on Pinterest as a team that 'gets it' and is using the platform correctly