I am thinking of building (or have it build) a social networking site with the possibility of back end management just like CMS, so administrator of site can add/modify things on the sites etc.
Plus I am also looking for plugin/widget container inside it, so developers/enthusiasts can add plugin or widget on the
site for free or can make money.
What technology you suggest can help it develop in quick time, I need a honest opinion and what can it cost me if
I want it to be build, any suggestions?
This is my first post here, I saw your question. What a great question to answer as I introduce myself to this community, this is right up my ally. I work for a web development company, we do this type of development every day.
It's possible to create the site using Joomla with many of the features you're looking for. You would need to develop the website in a scripting language like PHP to capture the user experience you're looking for. Joomla has some community tools that allow for submitting documents, this can be a file type of any kind. Check the "Joomla Extension Directory" there's probably one that does exactly what you're looking for.
If you want to make this into a business you may want to consider hiring a professional. What you described is called a portal site. Typically a portal site requires a decent amount of project development documentation to define the logic. I get the general sense of what you're going after, however it can range from $10K to $100K to start, for a veteran web developement company to work on a project like this. Without more detail (wireframe, technical documentation, mockup, user stories, use case scenarios, etc.) to make this definition.
If you want to formalize this into a business most clients I speak to are going to start with a POC (proof of concept) to get investor buy-in or create an MVP (minimal viable product), in other words, a version of your site that has most essential features to get started. But not everything.