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This would seem like a no-brainer, but it's not on your list so I would add effective email writing to your list. We all know a good personal email can go a long way toward building a relationship and ultimately, high quality links. If there is a website I am really aiming to earn a link from, I usually don't even ask until the second or third email, and by then I barely even have to ask.

Sounds to me like you haven't researched and targeted the right keywords. In my experience, long tail keywords convert better than high volume keywords. The trick is to target A LOT of long tail keywords to keep your volume and sales up.

I would setup a Google Adwords account and start testing there to see which keywords convert the best, and then target those same keywords in organic search. This way you can quickly get relevant feedback.

In my experience it really depends on the user is sharing the information and their Facebook privacy settings. If the user has everything open to the world, then it stands to reason that a Google bot will see these as well. Google has stated that Facebook and other social media sites have been given more weight to their organic rankings, but it is hard to say how much. Either way, I strongly suspect that this trend will continue.

ALSO, Google does rank the influence of the user sharing the post. If a user has a lot of 'trust' then that share will be worth a lot more than say a new Facebook user. Something to keep in mind.