How important is a online reputation for academics?
Generally speaking, online presence is not so important with respect to your career advancement in academia. Your professional stature is based on your publications, presentations, connections, and ability to obtain grant funding. When you look for positions, you'll be judged primarily on these criteria; blogs and media appearances are a very minor factor, if considered at all.

That being said, it may be beneficial in other respects. A strong online presence can significantly boost your visibility to those outside the academic environment. This can lead to consultation opportunities, collaboration opportunities with those outside of academia (think business ventures), and possibly job leads in industry. To the extent you're interested in those, you may want to invest in creating a strong online presence to help you advertise yourself.