I was dating this very sweet girl until earlier today where she told me it would not work out and we should just go back to being friends.
We were in a short term relationship before another man showed up in her life looking for a shoulder to lean on.
They were friends and I trusted her so I said do what you think is right to get him back on his feet.

When he found out I was dating her he started spreading bizarre rumors about me that my friends shrugged off as 'stupid' or 'out of character'. My girlfriend however, having known this man better than she knew me, began to doubt me. I sat through what felt like an interrogation with her and two of her close friends and said outright that I didn't feel like I deserved this level of mistrust yet I was willing to answer any questions they had truthfully.
I found out later through one of her friends that it was him who was spreading these lies about me and decided to confront both him & my girlfriend about it. I said that I didn't appreciate being lied to and slandered and told my girlfriend I would rather she doesn't associate with him and that he can go his own merry way (The exact wording was a bit more explicit than this I must confess).

On social media (Tumblr/facebook, etc.) I noticed she had many photos with him, many more than we had together but I figured since she's known him for so long they would accumulate. She had taken more photos with him than with me since we started dating.

Three days ago she refused to answer the phone, text messages or any contact I tried with her and removed my photos from her Facebook. She told me via text today that she'd rather be friends and keep me in the friendzone as I'm a nice guy. No doubt in my mind that she's with him right now.

What can I salvage from this? I've been putting out the fires from his slander to anyone who doubted me and who I wanted to keep in contact with. I'm considering cutting all contact with this girl which is a shame since I am somewhat annoyed at what could have been a great relationship.

I think you did right by confronting him, and even her for believing him.
But you ruined it by telling her not to associate with him, and by telling him to get lost. You have no 'rights' over anyone in the world (who isn't your minor child), so you came off as a control freak, and a helpless one at that.
Damage done. Nothing to do, unless you want to wait for her to see him for what he is. Which she just might do, as long as you can keep your integrity and keep quiet.

How old are you all? I would be shocked if you all are over 14 because really this sounds so middle school.

Why would you want her back? If she was willing to believe something that wasn't true then she might not be worth the time. And if she didn't know it wasn't true then it sounds like she didn't know you very well after all.

My friend I would have dumped her and moved on to better things, like a female who never allowed the lies to influence our relationship in the first place.

I wouldn't be trying to salvage anything with an unwilling partner at all, but what seek to run like hell and disappear from her life forever.

Salvage instead your own dignity, and self respect, and cut this female loose and gain happiness instead of drama, confusion, and conflict. DO NOT ignore the fact she is easily influenced by another, or has allowed another to come between you. You also did more harm than good by demanding she stop this contact with him, when in truth you should have just ended the fake relationship, though you still have false hope she is on your side when she clearly is NOT.

You have the facts and that's where a good decision by you lies, not just with your feelings of hurt, frustration, or confusion. Obviously your love for her, is much greater than her love for you, so sorry you gave your heart to an undeserving, and unwilling female.

Take it back and protect it yourself since clearly she will not.

I don't think you did anything wrong, except unwittingly encouraged her into the arms of another man. You trusted her, which is also not a mistake, and then you tried to draw out the truth, which you did for the most part- and the truth being, the two of them deserve to be together.

I mean that in a sarcastic way. What kind of woman plays two men off against the other knowing playing that game would inevitably cause some sort of showdown.

What kind of man stabs another man in the back like that.

Like I said, they deserve each other.

One bit of advice I would add, is stop defending yourself from any ridiculous accusations. You already know the truth, and the truth doesn't need to be defended.