This is a great question - even though it's worded messily. The way I read it is;
How do I get people to interact with my brand new social media site?
Well, that's not going to be easy. First off, most major social media sites didn't start off Facebook didn't start off as Facebook. It started off by simply being a site where you'd rate a face! It just ended up turning into far more than that.
Even so, it had an "in between" step, of being targeted at a specific audience. Creating the need to be on there by fostering the fear of being excluded from a group.
The answer (part 1)
To start a natural, sustainable growth, you need a hook. Some reason for people to come to your website and use it. And it'll probably be something other than "to share stuff with friends", because that's already possible over on Facebook, where all their friends are.
What that hook will be is not something that's easy to answer, and depends on a range of variables such as market demand, advertising network, your development resources and much more. Basically; what CAN you build & advertise?
The answer (part 2)
Your question is focused on getting content on your site. There's three ways.
- Incentivize content creation with rewards for posting
- Buy content from copywriters or throw some cash at your friends
- Create content yourself
That first one is probably the best option, but also the hardest to pull off properly. It could function as a hook on it's own, if the rewards are prestigious enough. Rewards could be anything from "access to a special feature on the site exclusively for beta users" down to monetary rewards. It's all up to you.