When you consider how relationship development is at the heart of social computing and enterprise 2.0, it should be natural to consider the career and leadership development of your employees in this context. This opens up new areas of thought into what it means to influence and lead others through an entirely digital medium rather than when you have a face-to-face leader. In my Forbes article (on Apr 16), I describe it as digital eminence to differentiate from one's leadership activities and capabilities through non-virtual environments--often amusingly referred to as "in real life", IRL for short).
The best way that I have found to describe it is in terms of how do people understand, appreciate and recognize your expertise, knowledge and skills through online interactions. This could be anywhere online, even email and chat, but it becomes more visible in social computing environments. I also like to separate this idea from personal brand building. While conceptually you are actually bringing out how you are different and significant from other people--even perhaps Seth Godin's notion of a linchpin in your organization--brand building also harks of self-promotion and ego-stroking. Digital eminence emphasizes what others think of you and your abilities, which may or may not have anything to do with self-promotion.
A second danger is in trying to quantify what is essentially a qualitative assessment. We should be very careful in considering number of followers, friends in your network, or quantity of posts as an indication of one's digital eminence. When you consider eminence as how you stand out, essentially a comparison versus the aggregate group of others in the same field, it may be seen as a ranking. Similarly, such quantity metrics also reinforce this ranking and rating approach. That raises lots of ethical questions when you look at it per individual.
That aside, the real question is what are you doing about sharing your expertise and skills with others? By doing so, you are building your digital eminence.