There are companies that specialize in submitting crap to digg, reddit, even here on HN - those are all paid links.
Infographics are a marketing tool sites use to generate links - you pay your money, someone designs it and submits it to social news sites to generate links back to your completely unrelated website. Those are paid links too, and not necessarily the aforementioned ones.
The mainstream blog networks are virtually content farms these days churning out summaries designed to do nothing more than reinforce their search positions for companies and products etc.
There's a big market for selling and swapping links, any forum for webmasters of any kind is going to have that at play.
Then there's flat out spam - last year I watched a site get all the way up to the top 10 for "free online games" with nothing more than blog spamming software.
There's all the sponsorships, endorsements, paid promotions, reviews with incentives, referral/affiliate stuff, etc etc.
So many ways money can change hands to cause a link to appear on a site.
Meanwhile there's a massive amount of the internet that is not going to get any legitimate link love, ever.