Zipkin depends on the services in the stack to be able to capture trace identifiers, record and send to the collector timing information, and forward trace identifiers for outbound calls. This is easy in Finagle as all of this logic is already abstracted away for most of the protocols. Note that Finagle is open source so its ready for use today.

If you wanted to record and forward traces inside your own services, this is relatively straightforward (though not super trivial) to do. There are other implementations beyond the Finagle one in development by the community - I would suggest hopping on to the zipkin-users Google Group.