Based on my analytics stats I got traffic from forums but not that much and I think you can build brand and authority too because of the sig link everytime you post.

try to submit the page in GWT "Fetch as Google" and submit to index, wait for 1 week to see any changes in Google search results.

Have you tried reading the meaning of it in the SEOMoz? They have great explanation here:
http://www.seomoz.org/learn-seo/page-authority
http://www.seomoz.org/learn-seo/domain-authority

Are there free tools that I can use? I need to explore and learn how to use this stuff.

SEO is part of an online marketing campaign, promoting your website and adding authority to it to be visible in search engine results pages.

Social Bookmarking is use to manage, store, share, organize and serch for bookmarks or resources online. So if you like an article or a story and might want to store it or share you can bookmark it first for future references.

I agree with opensiteexplorer.org you can also compare your website's authority with your competitors, the only problem is.. limited to 1 search per day I think if you are not a Pro-member.

Most probably there are following updates after the 3rd Penguin update. Google hates spammers and SEOs because of manipulating the organic results so expect the next update.

Having duplicate content will have a huge impact in your website, your website will not rank in search results and you will lose traffic for that. Better rewrite your content and have it check in copyscape or any article checker to make sure it is not duplicate.

SEO isn't dead, just yet, but the whole site marketing landscape is changing fast and unless you start adapting now the harsh truth is that your future will be very bleak indeed.

I agree, we just need to learn how we are going to adapt the new changes especially the updates in google algorithm. SEO is still part of the online marketing, different strategies can be use to make the website visible and have higher traffic/sales.