First of all, I really like what SE.QF community currently is now. It generates very high-quality questions and answers than any other places. I am really glad to find this site through R-bloggers, and hopefully I can contribute to it.
I doubt you will like the following idea, but since currently there seems no other efficient way to market this site, let me just throw it out and leave the judgement to you.
I am thinking to market this site through many linkedin groups. There are tons of quantitative finance groups there but have very low quality in their discussion. Mostly spammed by head hunter, sales people, and news. Despite of low-quality content, there are still tons of members. But seriously I bet people was subscribing for group content like SE.QF, not those spammer. Introducing SE.QF there will definitely attract a lot of new blood from there.
However, even myself is hesitant to do that. I believe this site currently has high intensity of truly professional quants. New bloods from LinkedIn groups will definitely lower the pureness of this site. That's why I want to say something here first.
One possible solution is to be very selective. A good example is the SYMMSS ran by Attilio Meucci. Nice quality of discussion and questions.
However, I don't know many group like this. If we really need to boost the traffic, we probably still need to advertise in big groups and will attract many not very professional people.
BTW, instead of broadcast advertising, an alternative is to answer people's questions there with a link to SE.QF (hey! your questions has been answered here), or even suggest them to ask here if nobody there answers (very common).