(i) when a Bloomberg user uses the terminal to lookup securities and other economic/financial information, they don't want to be open about it (trading strategies, investment focus, etc, are well guarded secrets and become real competitive advantages),
(ii) most of the firms that use Bloomberg Terminals have Twitter and other "social" networks blocked by firewalls.
Correction: given that it is a read only, "curated" list of users, I do believe it will be used. I was skeptic about the ability of being able to tweet, but it's been made clear that tweeting won't be permitted.