“You really need to be clear about what your executive sponsors are looking for. The common response is ‘increased employee engagement’. For me ‘Engagement’ means open and transparent conversations. Therefore I would advocate that the breadth and depth of conversations on Yammer should be a priority over simpler activity measures.”
What should we measure?
"Social cohesion, it simply measures the degree to which your interactions are reciprocated by others in your network e.g. you reply to my posts and I reply to yours.
By focusing on amplifying social cohesion, you will generate a more sustainable collaboration performance.
The choice is clear.
If you define engagement as engagement with the technology, then activity measures will suffice.
But if you define engagement as people engaging with each other in productive collaboration, then social cohesion and related relationship centered measures are essential."
Another really useful post cited in that article is this one: The smoke and mirrors of Enterprise Social Networking metrics
Both are worth reading.