How Would you spent $5,000 to promote your site? SEO? TGPs?

I'd like to hear some of the vets on here share how they feel 5k would be best allocated to promote a site. Its a free social dating/tube site (all user uploaded videos, not illegal dvd rip's like bigger tubes) It's approx. 1 year old now.

Has reached 10,000 on Alexa, currently see's around 70k uniques daily (around 500k page views a day)

This bit of traffic has been generated with ZERO Seo, so I'm sure that might be a great start. but I'm open to recommendations regarding this and any referrals of SEO firms that are TOP Notch in what they do.

The site gets all its traffic from other socials and users posting videos basically all viral marketing.

I've tried buying traffic from choker and traffic holder but it seems like the hits are invisible. From what I can tell in the stats, the users hit one page and immediately leave or never wait for it to load. (this is just the case with purchased traffic) Since my site is free, it's hardly worth it to purchase traffic or do anything like ADWORDS.

Im now finally able to re-invest some of my earnings into the site and want to do it wisely. Thanks gang!