I guess the first question is whether this is really a problem. Many companies have more than one instance of an ESN, often caused by the grass roots nature of early social experimenting. Many of the current ESN tools are available in a free version so there's a very low barrier for bringing them into a company. It's also likely that those networks are in different departments, not "officially" sponsored or approved, and only adopted by a limited number of employees. At the point that the company reaches a higher level of maturity there's generally an effort to consolidate on a single, approved ESN and to kill off the other networks. How successful that effort is depends on a bunch of factors including how the approved ESN is implemented, which tool it actually is (does it really meet the needs of the business, is it a good user experience, were the users actually involved in its selection or was it forced on them by IT, etc.) and do the other tools have to strong a hold on a specific population of users. There is certainly the possibility that the effort to consolidate might not really eliminate all of the other existing networks.
The other source of the network sprawl could be from networks embedded in other systems. In some functional areas there is at least an argument that a specialized network might be a better fit. If there are special feature requirements that are captured by the embedded network if might be in the best interest of the users to implement it in that business area. Human capital management (HCM) systems for example, seem to be on the forefront of this trend, both Oracle Fusion HCM and Workday HCM include an embedded social network. Oracle Fusion HCM is solving the issue itself in a future release as it will replace the embedded network with its standard ESN product, Oracle Social Network (ESN). The trend though seems to have some momentum and might continue to add to the proliferation of ESNs and thus add to the sprawl.
So what's the solution to network sprawl. At the simplest form, integration is the key to dealing with ESN sprawl in most businesses. It's not unreasonable to expect that many businesses will find use cases for more than one ESN tool, particularly as more tools become embedded inside other enterprise software. The ESN for many companies could really be an interconnected set of networks that solve different business problems. From a best practices standpoint though, I think that businesses will need to establish a social backbone with an overarching and very feature rich ESN to be the connection point for all the other networks. In fact, there is also a need to include communities that exist outside of the corporate firewall, like customer communities, partner communities, supplier / trading communities and maybe even some public social networks. Yammer's CEO, David Sacks referred to this concept as building an enterprise social graph. What ever you call it, each business will have to define its own set of communities and networks, and ensure that each is properly integrated to prevent the proliferation of organizational and system silos.