I have a very popular site on Internet with Alexa rank between 200-300

I am looking for opportunities to promote my website on social network and tap into the users of Orkut, MySpace, del.icio.us etc...

Let me know if there is any company who has managed to promote their website successfully in these networks without paid advertisements.

Promotion on Social Networks (Social Media Optimization) is the next level SEO.

Regards
Praveen