Google claims their users click (organic) search results more often than ads, essentially rebutting the research cited above. A 2012 Google study found that 81% of ad impressions and 66% of ad clicks happen when there is no associated organic search result on the first page. Research has shown that searchers may have a bias against ads, unless the ads are relevant to the searcher's need or intent 
Forum comments work much the same way as blog comments, except you tend to lack the base post from which to play off. Instead, you need to invest some time into getting to know the culture of the forum, the prominent users, the rules, and the discussion flow. If people generally post one sentence at a time, adding a 3,000 word post will be excessive and may be mocked. If people tend to post lengthy discussions, short posts may have a negative effect. And, like Reddit, some sites may be very rabid about enforcing no advertising.
Facebook ads are a great way to get highly targeted traffic to your blog (landing page, fan page, what ever). Although not a free traffic source, it is a great one. The price is not very high and I think it is pretty acceptable considering that you can choose demographics of Facebook users that will see your ad. I had great success getting traffic that converts on one of my niche sites.
Interviews can be a source of some decent traffic, both in giving and hosting them. On the giving side, you’re building your reputation and you’re gaining links, as the places who interview you publish a link to you as their source. On the hosting side, you publish links to influencers you interview, who have a decent chance of linking to you as a “check out this interview I did” comment somewhere. They have an interest in promoting it, after all.
I am actually using a free service called Cloud:flood created by Glen of the ViperChill. I had great success with it, making a file available for download (I shared a list of 140 blogs worth following). It has two options, a person can RT the download that will take new people to a post where the download is (or what ever landing page you want them to come to) or they can share it on Facebook instead of tweeting it.
Would you mind sharing your stumbleupon user name with me? I would love to follow you and your shares! :) I am beginning to get more active on SU and it is definitely paying off. I have had a few posts in the last several weeks that have brought in 600-700 visits each. :) In regards to your "challenge" I am going to try out paper.li I will let you know how it goes.
Haven’t heard of search engine optimization yet? Search engine optimization (or SEO for short) is initiated to help boost the rankings of your website within organic search results. Some examples of techniques used to boost organic SEO include using keywords, building local citations or mentions of your business across the web, and writing content that’s valuable and relevant for real people. The goal is to have unique and quality content, a user-friendly website, and relevant links pointing to your site. These are just a few of the things that can influence the rank of a site within an organic search.
The Google, Yahoo!, and Bing search engines insert advertising on their search results pages. The ads are designed to look similar to the search results, though different enough for readers to distinguish between ads and actual results. This is done with various differences in background, text, link colors, and/or placement on the page. However, the appearance of ads on all major search engines is so similar to genuine search results that a majority of search engine users cannot effectively distinguish between the two.
Organic traffic is the primary channel that inbound marketing strives to increase. This traffic is defined as visitors coming from a search engine, such as Google or Bing. This does not include paid search ads, but that doesn’t mean that organic traffic isn’t impacted by paid search or display advertising, either positively or negatively. In general, people trust search engines, and sayings such as “just Google it” reinforce that humans are tied to the search engine. Thus, paid search, display, or even offline campaigns can drive searches, which may increase organic traffic while those campaigns are running.