Welcome to JavaRanch Ajay,
This is a really broad question. Trust me, you can write a book on it. But I would say the base of everything lies in what you want in your social site. Take a paper and a pen and list down all the things that you want in your social site. Decide the flow of your site, and then write down what you need to do in order to create that flow. Draw on paper what functions each page would have and how those pages would interact. That would give you a better idea of what you are trying to make.
What technologies to use is totally dependent on you
Should I choose hibernate and Spring or just use jsp-servlet would be good enough? How can I keep it simple & light?
Again that totally depends on how big and complex you want the site to be. Jsp-servlets would be good enough to build almost anything, but i suggest you use a framework to make it more maintainable and extensible later on. Using a framework would help you a lot with that. And you dont have to necessarily use Spring MVC, you can go with other simpler framework like Bear Bibeault's Front Man, if you think that spring might be an overkill for what you are trying to make.
3. Which database should I use?
Again, depends on how much traffic you are expecting. A thousand users is not a very big number, you can use any database. But again that depends on the functions that you provide. If the functions you provide need heavy database resources and querying, you might use a more robust database. But if you are going to use hibernate or any other ORM, choosing a database is not a big issue, as you can easily move to any other database later on if you are not satisfied.