Whatever people may say, it’s still the hot issue of enterprise social networks. Considering ools that that are not processing tools strictly speaking, benefits have to be found on the new way of doing things they make possible rather than in the tools themselves that are only enablers. As I wrote here, benefits are not on the cloud but in the operational reality.
This said, the answer is still hard to be found.
So we may follow Forrester :

I found the list of direct and undirect benefits very exhaustive and clear. But is that enough ? No. If we can explain, for example, how intangible assets contribute to value creation, we cannot explain in which measure. Let’s consider the CISCO case. Chambers can give a backed up by figures ROI in terms of capacity to drive projects and in financial terms, but I’m not sure that when the decision was made had any figure he was automatically sure he would reach.
You can find the "original" french version of this blog here

