How to encourage your community to contribute more content
There are plenty ways to get people to contribute to your site, the key is finding something you can offer them, that doesn't cost you much time or money. Here are a few examples:
Q&A sites, and social media sites, often convince people to contribute content by rewarding them with access to features, giving them power, or offering points of some sort.
When you post something on Facebook, people can "like" your post. This is a form of giving points. People feel good that other users enjoyed their post, and this inspires them to post again. It costs you nothing but the time it takes to develop the feature, but people are willing to trade their time for these imaginary "points".
When you post on StackExchange sites, overtime you unlock features (eg: ability to see all upvotes/downvotes, post answers on restricted questions, edit questions), and this makes people aspire to unlocking more features by playing off human ambition. Video games use this same technique to keep people playing. Again, it doesn't cost you any money, just the time it takes to build these features.
On many forums, your post count is a determining factor when asking to become a moderator, or join certain groups. People like to feel like they are not only part of something, but integral to it's success. The best part about this is that you're not just getting content, they actually help run your site.