Before investing the money in some developers, i would ask if there is a need for this
new social network. FB was not built over month. How you are going to add more features when in its simplest form it will take over a year to create it. Not to mention that at that time
technology will have shifted and enthusiasm will have been burned.
Of course if the network is very niche and there is a need, adding the smallest set of features
just to go public and get the work out, the budget is good; which again depends on the origins of the development group or person (country) and again you get what you pay for.
My advise would be not to use any freelance sites and go with some reputable person that has very good referrals and can take this project to the end. If you can find someone around your location it will be a huge plus. So to wrap it up, I believe you can get away under $5K.