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.

The reality is that I'd never just use StackOverflow to find candidates, so these are people who have other ways to find out about the positions, and apply.

I'd rather see the candidate spend the money on an ACM membership (or similar professional development) than spend it simply trying to reach me.

That said, I think I now have a pretty good idea of the value proposition as it might exist for other employers, but it seems clear that the equation doesn't balance for me. Thanks for clarifying your argument, I appreciate it.

Joining ACM: just a plain bad idea. Only value is older papers not available ungated on citeseer or elsewhere.