A brief account on what a reusability KPI is in SOA.
Reusability refers to the number of times that a software asset is used over a period of time by other software assets. The term is typically applied as a prefix to a specific software asset as in "Service Reusability".
Service reuse is typically understood as the number of consumer applications and/or projects that use a given service during a period of time.
Use of a bottom-up service development strategy: the main risk in a bottom-up service development strategy is precisely that services may not be required by any consumer applications at all.
Consumer applications have several exclusive features: The reason why applications get commissioned in the first place is that bespoke functionality is required. It follows that services created in response to functionality required by consumer applications (top-down service development strategy) are likely to be useful only to such consumer applications.
Use of an ESB as a front-end/back-end separation layer: Under this scenario, all functionality required by a consumer applications is implemented as services. Given the previous point, the reusability ratio is likely to be low.