Looks like many contribute to StackOverflow so that they can get a better job (via Michael Tsai):

After finding a new job, users contribute 25% less in reputation-generating activity on Stack Overflow. By contrast, they reduce their non-reputation-generating activity by only 8% after finding a new job. These findings suggest that users contribute to Stack Overflow in part because they perceive this as a way to improve future employment prospects.

Look, I don't think that's necessarily evil or anything, but it is a shame, if true. You should, I think, feel some sort of civic responsibility to give back to the site whose collective knowledge is very likely making you more successful in your job than you could have possibly been ten years ago. I mean, honestly, how many Jon Skeet answers have taught you something insanely arcane about .NET that you never would have stumbled into yourself (eg, if you're a .NET coder, etc)?

At the same time, I can think of a few reasons why this might happen that aren't as idealistically disappointing as feeling the site boils down to a giant portfolio for most people. When you take a new job, you obviously likely move away from a codebase, maybe even an entire stack, that you're familiar with, which often moves you from a position of knowledge mastery back to one of a beginner. I like to think I can get up to speed on most codebases in 2-3 months, and that's probably true-ish, but even then you've got those rare outliers (it was QueryOver in my last major new codebase) that you might not get to for a year or more.

Anyhow, I guess there are two takehomes for me. One: Come on, folks. You take; now give. Two: I need to read the study to see if a conclusion of "they perceive this as a way to improve future employment prospects" really is warranted.