Social Bookmarking for Programmers?

I am a new programmer, really just getting started. I don't know too many people who program around here so I am not sure where to take this question. I will try to express it as a proper question and not a matter of opinion or debate.

I just am wondering - what sites do programmers use for social bookmarking? Do any of these these sites have a significantly larger user base than the others?

Thank you.