We have shared some of the most popular social marketing channels with you. However, there are a lot of other social marketing channels that you can consider too.
1. Write a response or comment to a topic with your solution and company link.
2. Create your own topic or page related to your product.
Note: The channels or tricks you see here might not all be applicable for your startup or situation. However, see them as great examples to help you deduce equivalents that are suitable to your specific market or location.
Hacker News, Reddit
100 Million+ Users
Facebook, Twitter, LinkedIn, YouTube, MySpace (though they're fading)
25 Million+ Users
Reddit, Flickr, Yelp, Wikipedia
10 Million+ Users
Tumblr, StumbleUpon, Care2, Slideshare, Scribd, DeviantArt, Digg (fading), Delicious (fading)
Posterous, StackExchange, Namesake, Quora, FourSquare, Hunch, Forrst, Dribbble, Skillshare
Taken from: http://www.seomoz.org/blog/social-media-marketing-facebook-twitter-arent-enough
A good example of a company doing good content marketing on social marketing: http://www.eastlinemarketing.com/social-media-marketing