Here is a repost from my answer on Quora.
How difficult is it to create a social networking site with capabilities like LinkedIn and Facebook?
When Facebook and Linkedin first started. Their sites wasn’t as complicated and sophisticated as they are now.
Your goal is not to start with a project that is as advanced as they are now. But to find a niche that you are able to build from the ground up. From simple and easy to understand. Make it useful for people and let it grow organically.
What you need is social proof that people like your product and has the potential to reach critical mass and exponential growth. From there, you can invest or find people to invest, and then add more resources to grow your site to something similar to Facebook and Linkedin in terms of their size and complexity.