I am a huge fan of stackoverflow. I am constantly on it and I contribute as much as I can. I can't tell you how excited I was when I reached 10,000 points and was granted access to mod tools. For me I felt like it was natural for any programmer to automatically be part of something this amazing, but it is not the case. I am surprised to find that most if not all of my coworkers don't even have an account.
I encourage my coworkers to sign up at least and see for them self but they are not very excited about it and I try not to insist. It reminds me of my friends bugging me to create a Facebook account. And my answer was always No!
If you read most tech blogs when the subject is resumés, your github profile is the first thing you are asked to include. I would say your Stackoverflow profile could be much more valuable. Anyone can memorize how link lists work to impress their interviewer, but seeing how you actually help solve someone else problem says much more about you. And it all goes in your careers profile so you can have employers contact you directly.
If you have been using Stackoverflow for a while and haven't contributed, well what are you waiting for? If you see a question you can answer or think you can answer please do. If your solution is incorrect someone will let you know, and either way you will learn something.
Knowledge is to share.