I think this is because of the inherent inequality that exists at work. In a typical social network, everyone is essentially "equal" when it comes to who you know and interact with. At work, you necessarily are very closely involved with people who hold significant authority over you. Because of this, you have to act incredibly more guarded at work and on its related networks.
For a social network to have value, you need users who are compelled to frequently submit content. By adding a power structure, users have to evaluate their decisions, which can generate anxiety or extra work, leading to a drop off in contribution rate.
It's one of the reasons teenagers ditched Facebook once their parents joined.
