What do you need to develop your very own social networking site from scratch?

[Tim]

Well to start with I would say you would need to pay to do market research on your target audience and see what they want.
Then get an office set up with a dedicated internet connection and several servers to host the social networking software and all the databaes for users, email servers, etc.

Then hire out programmers to write the whole social networking site and then test it out on users for feedback / to identify holes in the system, etc.
Then go back to the design and implement any changes, modifications, fix the bugs.

Then spend a hell of a lot of money on publicity to get the site popular and see if you can get contracts with advertisers to get money or sponsorship deals to try and make some cash.