(GO DOWN FOR TL;DR *too long; didn't read* Do I believe they exist? Hell yea. Why? Secret societies, cults, shady organizations, etc. have existed for years. One could view the Knights Templar, old high-ranking church officials during the Reformation, among other people as the secret societies of their eras. Most people blow off anyone who would believe these things because people want to believe that while humans can be cruel, they wouldn't be so heartless as to control, manipulate, and basically brainwash our world into wars and facist states so they can step in as heroes and gain more wealth, fame, and power... Right? They already have billions on billions, why crush their own species so they can get more? Aren't we all a human race? No. Humans are animals and people just need to accept that. Do wolves look out for other wolves? No. Just their family, their pack. And if their pack doesn't do as the other wolves please, they may turn on him too. They take territory and will kill any wolf that gets in their way if necessary. Even though humans are cerebral, we still have a primal brain and minds that can be tainted with wanting more, just like animals.
But we don't fight like animals do we? Yes, we do. But instead of tooth and claw, we fight with guns and tanks. Instead of alphas and dominant males/females, we have cooperate leaders, presidents, prime ministers, and royals. You and I are the omegas to the governments, cooperations, banks, and other big-wigs that will stomp on anyone to keep their power. Thing is, it's getting harder to keep us all tame. Political rights leaders and activists spring up left and right because people aren't as idiotic and poorly educated as we used to be. Questions are no longer rude or a bad thing and education is seen as necessary by more and more young minds. Being the slacker kid who doesn't take school seriously isn't cool anymore, it's annoying.
But, unfortunately there's nothing anyone can do about it until society crumbles again and remains of documents and record of our current secret society is found. It's a constant struggle that never ceases. There will always be power hungry people willing to do anything to get on top. It's unfortunate, but it's just human nature. If you can't believe me, think of this: why do so many activists, people with different views and ideas, powerful, influential people with loud and outspoken voices, die so soon? Princess Diana, Prince, Martin Luther King, John F. Kennedy, and the list goes on. JFK and MLK had some ties in to the CIA plotting their deaths but someone had to commission the order. And the guy who did had to have someone tell him, and someone else tell him, until we get to the beginning of command.
Basically, people are greedy and manipulative and are willing to do anything for power. They'll even shape the world so it benefits them.
Secret societies are absolutely real and are a vital part of American history. Only the people involved truly know the realm and overall scope of the decisions made, but there are definitely organizations that have had their hands in major world decisions. There is enough information in the public domain that provides proof of these organizations that it would be ignorant to believe otherwise.
I am currently a member of a secret society. I won't say which one, or it won't be a secret anymore. The point is, I'm living proof that secret societies exist because I happen to be a member of one of them. I think you should join sometime. We play bingo.
Throughout the history of mankind there have been secret societies, organizations and cults. The question should ask "do you believe there are secret societies in existence today?". My answer would still be yes, because there are always those among us who want to have something different and apart from what the average person has access to. Some are able to achieve this with wealth, enabling them to purchase rare items. Others may achieve this by forming or joining a "secret society".
I don't think there is any good evidence to suggest or support the belief that secret societies are out there in significant numbers. There will always be people doing stuff in secret or otherwise in a shady fashion, but to think there is a widespread society is a little silly.