I think you're confusing professional presence with general presence. Having a blog where you post solutions you've found, posts of IT-related articles, links to a StackOverflow profile, and GitHub contributions would be great (these are just examples, not a check-list and not valid for any/all "IT" professions). Your employer doesn't and shouldn't give a damn about your twitter or facebook. Even better, in states where they can ask for your password, it's even better to not have one at all! Or make a FB profile that has nothing but a nice profile picture and the most locked-down settings FB allows you to have.