A structured action plan for a social networking site could be:
A: Research The Idea and the Business Model
A1 - Create Mockups of your idea (e.g. using Balsamiq Mockup or other tools)
A2 - Use the Mockups and develop a questionnaire about your idea. Create a landing page.
A3 - Conduct 200 offline and online interviews to potential users, to get feedback on your idea. Show the mockups to offline users and send online users to the landing page. Use Mturk.com, AYTM, Pollfish and other services for cheap feedback on your idea.
B: Set-up A Team for the MVP
B1: Get a technical co-founder or two. You will need at least a couple of developers for your MVP (back-end/front-end and designer). If you cannot get a technical team together and you want to pay for your MVP, then get someone to write the technical specs for your project. Use these specs to recruit freelancers from elance/peopleperhour/odesk etc.
C: Build the MVP
C1: Do the project management for the MVP.
C2: Continue testing the marketing channels and driving early adopters in your landing page, so that you create an email list with your first X thousands of users.
D: Get Your First Users
D1: Use social media, paid ads, PR releases etc to drive traffic to your site.
E: Prepare for Next Steps
E1: Test the business model. Test conversions from visitors to members. Test analytics such as engagement time, times of visit per day etc.
E2: If you have found a way to grow every week you will probably need to raise some money. You have to prepare for pitching to investors and raising your first round.
Let me know if you need more help and good luck with your project!