Nik, this is an interesting point. While I am an advocate of claiming one's online identity and pushing the boundaries of professional development, I have come to learn that, much to my surprise, many of my graduate and continuing education students do not blog (to such an extent, that many of them wonder why I advocate it so much). To get them started, I often recommend that they begin by discussing and processing what they are learning in class, and how they can envision using it in their work. While I see it as a step toward an online portfolio, I think this concept is still too new for many people.
For all our talk about the online teaching and learning opportunities, I am often surprised how great the gap is between those who actively engage in multimembership, and those who do not.