Since I think inviting bloggers to write a blogpost about a certain topic is a great method of generating ideas, I'm respond with this blogpost. Let me answer it from the perspective of nonprofits working for international development.
The word Return On Investment makes me feel I have nothing interesting to say on this subject, so don't expect any neat calculations from me.... .I do think it is possible to monetize the effect of using the social web for development organisations but that's not my specialisation. And when monetizing, it could be the challenge not only to measure the direct cost reduction (like the amount of money saved because employees use free skype calls to talk to partners instead of telephone!) but also the indirect benefits like improved relationships and changed power relations between partner organisations in the south/east and development organisations in the north. (mind you, the term partner organisation is now in common use, but that name may conceal the difference in power at work in the collaboration between the two).
That brings me straight to the heart of the matter: I believe that the social web can lead to changes in the way of working at various levels. For the northern development professionals it means the opportunity for faster connections between people with the right expertise, and hence improve quality of the work. You don't limit yourself to select the consultant you know, but you use linkln to select from a wider range of choice. I wouldn't have been part of this group if I had not started blogging. So the tools enable new connections.
Another level the social web can bring about changes is towards empowering southern organisations, think of southern NGOs who would use the social web to exchange information about their partner/donor organisation. That would make them more aware of where they stand and what's possible and would probably widen their scope for action. Common use of web2.0 tools can bring about more equal ways of collaborating, see the story of how milieukontakt's use of wikis makes collaboration with trainers in the east more egalitarian.
But there is an assumption here: that we are all working with the social web functionalities. I also believe that it's a whole new way of working and networking that is not yet a common habit yet. Maybe individuals are ahead of their organisations. Last week I listened to a webcast about enterprise 2.0 where the directors agreed that web2.0 started from the consumers side and that the enterprises are following. I think this is similar in the nonprofit sector where individuals may be ahead of the organisations. So welcome organisation2.0!
By the way, if you want to see more blogposts and ideas on this topic, watch the netsquared blog.