First two parts of the answer:
One part of the problem is that this site has substantial overlap with those other sites. Many questions which should be asked here, are instead asked there - and mostly answered - even if often not correctly (ma laasot, programmers dont always know as much security as they think they do :) ).
So the first thing we'd need to do - get the SO/SF crowds to ask their questions here.
I've gone to many questions, and left a comment saying they'd get better results here - sometimes it works, mostly it doesnt. But that is definitely the first place to go, since most of the users there don't even know about IT Security SE.
The second part might be harder - the types of people that are not on either site - e.g. CISOs, Risk Managers, consultants, etc. These would also have much higher level of questions...
Near as I can see it, @Robert's advice (no.7) still very much applies - we need to get the word out, send the link to all our (professional) contacts, existing communities, conferences, etc.
Personally I already spammed all the infosec groups I'm a member in on LinkedIn, which I think helps a bit... Also sent the link to many clients, colleagues, and other people I work with. I suggest you (and everyone else here) do the same, if you haven't already.
It's also a good idea to give a short talk on this at any industry conference you can, e.g. local OWASP chapters etc.