twitter fb linked... ads

Hi,

I'm not a social media guy, but I need to promote my websites, and I have limited options when it comes to getting targeted traffic fast, and the reason for this is because SEO is expensive and time consuming, while Adwords is expensive and complicated.

So, I have to look into other (cheaper) alternatives, and I (think) I have found a few: facebook ads, twitter ads, linkedin ads.

And my question for you regarding these 3 platforms is: how does each one compare to the others in terms of cost-efficiency, results, user-friendliness, and requirements. More specifically....

1) what is the minimum amount of money I can start with each platform to try their service?
2) do I need to have a member account with these platforms in order to use their advertising service, or will an advertiser account do?
3) which one is the easiest to learn and master?
4) which niches/industries convert easier on each platform? I know that these are big companies with billions of people showing interest in every possible industry you can think of, but those of you who have tested these 3 platform know for sure which niches convert better, and which ones don't. So, can you please share a little of your experience so that I can get an idea of what should I work with?
5) what is a typical conversion rate for each platform?
6) how much is the CPC on each platform? Again, I know that CPCs usually vary from one industry to another, as well as based on many other factors, but still, are the CPCs in the range of cents, or dollars per click?
7) do these platforms only offer PPC advertising, or do they have other options as well (like, for example CPM)?

If you have personal experience with advertising on these 3 biggies could you please share your tips?

Thanks in advance!

John