Serious SEO link building

I've been running a politics forum over at http://youthdebates.org (main URL http://youthdebates.org/forums) for over a year, and whilst we have thousands of users and daily posts, almost everyone finds us through Facebook. We have only 40 backlinks according to Alexa, and our search traffic is quite low as a result.

I'm considering spending some money on a serious SEO specialist, but I mean serious. This would only be for manual link-building work and no on-site work would be performed. It will take a very good proposal to make me consider, so please put time into it and be honest. Provide examples of past successes with evidence if you can.