Where are Share Classes defined in an LLC. For example, if I look at facebook's constitution at the link below, there are no definitions for share classes. I was thinking there would have been such a definition as Mark Zuckerberg owned special shares that entitled him to massive voting rights.
I assume you're talking about corporations, not LLCs ("limited liability companies"). LLCs aren't corporations and don't issue shares of stock, and in any event Facebook is a corporation.
Information relating to the shares of a corporation is typically outlined in the articles of incorporation, but practices and governing law varies by state and by corporation. Delaware, where most large US corporations are incorporated, requires the certificate of incorporation to list the total number of shares to be issued and the number of shares in each class, as well as information about the "powers, preferences and rights, and the qualifications, limitations or restrictions" of the share classes. See 8 Del. C. § 102(a)(4).
Facebook's October 2010 articles of incorporation can be found here, and information about its classes of stock can be found in Article IV.