They get pretty decent sponsorship from Microsoft as well, in exchange for XBox premieres. Again, definitions of success, but having a sponsor seems to be a good way to do it (sort of direct advertising money). They've also spun off into comic books (Dark Horse, relatively mainstream stuff), not exactly a goldmine there either, but some income (and some people do THAT for a living too!) Don't know how their dvd sales are (I know lots of people who have them).
I think for a webseries to really "make it", finding different sources of income is an absolute must.