a) Twitter can easily be missed based on the number that you follow.
Facebook limits your feed to those you most communicate with by default, unless you turn that off you may never see some of your connections again after a few months time.
Google+ doesn't seem to do that, just yet, and when folks comment, more folks will see your link.
b) Yes it is, and it's been very beneficial. Just want business pages to launch so I can do a page without fear that Google will dismantle it.
c) It's very high if you have a lot of followers, but very low if your friends don't share your links, or you don't have that many followers.
d) Facebook is designed to keep you inside of the walled garden, more and more they want to divert traffic to stay within their boundaries, which is why pages went away from FBML in favor of iframes. Putting your website content inside of facebook sounds great, until you realize that Facebook is advertising to your customers as well. Google is a discovery engine, designed to direct traffic flow to finding new things that might interest them.