How important is a online reputation for academics? I catch myself too often thinking about how to become more visible and present online instead of doing Great Research™.

But how important is it to be very visible online for academics and researcher anyway?

Here are my assumptions, numbered only for easy referencing.

The bad:


Tenure committees and such don't care for twitter follower  
Public bodies that provide funding don't care how many people have you in your circle
Colleagues might look down on you, because your blog posts are not scientific enough


The good:


Writing a blog can be a good exercise in writing about your topic
If you are looking for a job or position, it can be helpful iff your potential boss is also an 'onliner'
It makes it easier to network and getting in contact with other people who work on the same problems
It might help you to get work, invited to talks and book contracts in the normal world.
(For CS) Uploading and advertising your code will make your research results more used


Are my assumptions correct? What did I miss? How do handle your online reputation?
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.