When writing an academic essay, if I was to reference Microsoft or Facebook, should I write Microsoft™, Microsoft® or just Microsoft? What is the difference?
There is nothing wrong with Chris' answer, but I'd like to add two things.
First, (c) (from the title of the question) or more accurately ©, is copyright, which is not used with company or service names. So you would never write Facebook©, although if appropriate you may write © Facebook.
Second, ® - as I am sure you are aware, but adding it for completeness, trademarks can be registered with other bodies too, particularly outside of the US. For example, in Sweden, you would register with the Patent and Trademark Office (PRV). For EU-wide protection, the European Patent Office should be a good place to start. And so on.
It depends on the style guide you are using. APA format (and I believe Chicago) specify not using TM or R in the text. I suspect that most style guides say the same.
TM = trademark, which a company (or anyone else) can use without a registration with the government. It provides some common law protection.
R = registered trademark, which means they've been granted the trademark with the US Patent and Trademark office.