Now in the Apache Incubator:

Enterprise Social Messaging Experiment (ESME) is a secure and highly scalable microsharing and micromessaging platform that allows people to discover and meet one another and get controlled access to other sources of information, all in a business process context.
The ESME server is written in Scala and uses the Lift web framework to produce a browser-based user interface and also to expose a REST API.

Update: As Bill de hÓra points out, the usage of "REST" in conjunction with this API is a sad joke.