It's common enough that a bigger percentage of normal people than you'd think probably find themselves using it on sites at least occasionally (Reddit, etc.), even if they don't consciously think of it as Markdown or know all the particulars of the syntax.
The GitHub-style editors are nice in that when a user sees the syntax directly when they press a formatting button in the markdown-toolbar-element, it makes associating a hash symbol with a headline or two asterisks surrounding a word with bold text pretty straightforward. In that sense, they teach you through using it since the syntax is fairly simple for the basics for someone seeing it the first time. Use it a couple of times, and you'll skip touching the buttons--at least for the basics.
That said, I think a Writebook-style editor[0] mentioned elsewhere may have some advantages for people with less computer experience, but I don't think a straight GitHub-style one like this is bad in those circumstances. It's quite good, especially if you pair it with some instructions (whether they be in a modal or whatever have you).
I've had good success teaching markdown to a fair few non-techie people. At the end of the day it better serves as a machine-readable middleground between a user-facing WYSIWYG (markdown-powered) editor and the server responding to it.
