Hello Gustav, I have Pinterest, Twitter,Facebook and I am looking into Instagram. That said, I find both Pinterest and Twitter generate the most following and interest so far. I have not really exploited the potential that FB has, but I know I can generate more traffic and possible conversions through there. So I would say that both Twitter and FB rank the highest with Pinterest in third.

This is only the matter of preference. I do not use FB and Twitter at all. No one seems to be comparable with Flipboard but it is totally unpredictable. I had spikes 10 times the usual daily traffic in the site.

That's a tough question. Do you want to promote for free or do you want to set up a business account. I do mostly free promotion so I have set up a Facebook account where I post info about my sites. I get a lot of my followers from my personal account, but i keep the business and personal seperate. I am not good with pinterest, but I have found that lots of people look at my boards and pin stuff from them. Twitter is really easy to use and get links out. So, now that i have written all this. I will say that I like Facebook best, but I am a real novice.