I need to utilize a "Social Networking" bolt on solution...

I am the CEO and Founder of a company that will be catering to a large social audience, that could quite easily surpass over several million users in a few years….these users are already built into the industry that I am going after…The question is what software product, company, or web developer should I use to implement this social networking component. The website is built, but I have not incorporated a social networking solution, ideas please and contact information would be greatly appreciated.