CSF performs a set of governance functions needed to manage a system of services in addition to performing a "session" management function for integrating services to build composite services. You can think of the session management function as being akin to "a virtual instance of an ESB created for each aggregation or mashup." This allows CSF to be much more flexible because each aggregation is "context" driven to the aggregation you are building and not generic across a "common" bus per-se.
A follow-up question - can one assume that session mgmt is a combination of "ESB"+"run time goverance"+ orchestration?