I see several assumptions here. What is the problem your site solves? What specific user do you have in mind? What is your revenue model?
1. If you can't walk into the bathroom and look in the mirror and say "i'm starting XYZ product/service BECAUSE of ABC problem" then there is no reason for you to start a start up. You need to identify a problem and then envision the company that solves this problem, you are just envisioning a company(i'm starting a social network).
2. The most successful start ups are those whose founders CLEARLY UNDERSTOOD a problem they were dealing with and then worked backwards to EXECUTE a solution to that problem. You need to CLEARLY have a user in mind. When facebook started this user was a college student.
3. Who is giving who money? Are you gonna sell ads? Are you gonna charge a fee to users(not recommended)? Maybe at first there may not be much of this, but providing value will be #1.
A start up is like a science experience, there is trial and error and you follow wherever the trail leads. *You follow the market. *Most of the time when you start a start up, you will end up doing something way different than you predicted.
I cannot stress enough: understand you users, understand your users, understand your users AND understand the problem, understand the problem, understand the problem.
The better you understand the user and better understand the problem the better chance you have of creating a product or service that will be crazy successful!
You see the bridge that connects the user(the consumer) with the problem is the product or service. And you need to have the first two(user and problem) before you even think about creating the product or service.
Ask yourself what do people bitch about? What do consumers wish existed?