Yammer is opening up its microbogging platform. In "Yammer Community" people may now create a community without the requirement that an email address be associated with a particular domain.
ALT-J, Volume 17, Issue 1 March 2009, 33-47. This case study describes the use of a synchronous communication application (MSN Messenger) in a large academic computing environment. (peer support c.f. Twitter)
While Twitter has been less than forthcoming on how they plan to monetize their service, there is no shortage of ideas from third parties on ways to get paid for spending time with Twitter.