There's a debate over who's bigger, but that doesn't matter. We all know it's how you
use your fringe third party that matters.
Even though the Greens may be larger, I think the Libertarian Party is actually in better position to be a major third party. I think once they can get away from the image of gun crazy hunters in Michigan, they might be able to make some damage. Unlike the Greens, the LP has more of a foundation in a presumably alternative ideology, a "third way," if I may use the language of Sen. Clinton.
My beloved Greens, unfortunately, have a far worse image problem than the LP, and their solutions are really just socialism-light, with a pinch of Jeffersonian jargon in there.