Stackoverflow Careers is based on the premise that it's worth $5,000/yr to me, plus $99/yr/candidate for the candidate, to share that information.

Do you really see that much value, versus simply asking all candidates 'Are there any publicly accessible technical forums or mailing lists that you participate on a regular basis? If so, are you willing to share your username?'

I prefer the latter, because it's a question I can ask all candidates, and it doesn't cost me $5,000 to ask. Nor does it cost you $99 to answer.