I don't because of school. Instructors still mention it as a moderately relevant place to establish an identity though. Key ones are Facebook, Twitter, Tumblr, Flickr, Youtube, and Vimeo.

But I don't feel like keeping up on DA. Too much noise. Embed yourself into too many communities and you're spending all your times trying to keep up, or monitor your work. I'd also have to wipe my old account clean, and I'd rather just abandon it, and preserve it as an archive.