You realize that monetizing social networking sites has been extremely difficult for Facebook, myspace etc?
Additionally, starting up any major social networking site is going to require a huge chunk of resources that you can't get cheaply. Don't forget about the fees for employing a lawyer to write up everything from pirvacy policies to terms of service, written to fit where you run this 'business' from.
One of the main reasons why facebook, myspace, youtube, twitter etc. succeeded is because they had a large amount of resources available and they had to get this before any of these social networking sites were profitable.
Now, can you create a social networking site that is successful (less than 1% of social networking sites on the Internet are actually 'successful' in the way myspace, facebook etc. are) is something that has no clear answer.