Ok, so let's start with some background information on the people involved.

My aunt on my dad's side will henceforth be referred to as Aunt D.
My aunt and uncle on my mom's side will be Aunt M and Uncle M.
Aunt M is my mother's sister and Uncle M married into the family when he married Aunt M.
Aunt D and Uncle M go way back.
Aunt D and my stepmom also go way back.

Aunt D recently disclosed to me that Uncle M has been extremely sexual with some of their old high school buddies on facebook-so much so that they have had to block him.

I am extremely close to Aunt M-closer than I am to my mother. I'm about to get married soon and Aunt M is always giving me marriage advice and using hers as an example-it kills me to know what he's doing behind her back in spite of his self-proclaimed devout christian beliefs and the two innocent girls they fought so hard to adopt when they couldn't conceive themselves.

I normally don't lean too heavily on rumors through the grapevine but, since my Aunt M and Uncle M lost a baby a few years back, Uncle M has been an alcoholic and only recently came off his anti-depressants. They have a very unbalanced relationship in that Aunt M does all the work while Uncle M ensures that it stays that way while he watches television.

They've been married about 12 years now and after everything that they've been through to have a family, my Aunt M is finally happy. I'd always thought Uncle M was too but now I'm not sure.

What do I do with this information?

Mind your own business and don't get involved comes to mind

Yikes that is a toughie. Hmm infedelity, to tell or not to tell.
Do you think Aunt M possibly knows about Uncle M's little Facebook issues?
It's hard to butt out when someone you love is being played for like a fool. If I were Aunt M, I would want to know. If she truly is happy in her marriage and loves Uncle M I am sure they would find a way to work things over, such as counciling, him getting back on his meds, AA, etc etc. I couldn't imagine being in a happy relationship where I was doing all the work while hubby sat around drinking beer watching the tube... When I was cheated on, I was very embaressed, and mad that it had gone on for so long with MY friends knowing what was happening and not telling me. I was so embaressed that they knew what he was doing, while I was professing my undying love for the jerk. Also a good friend of mine was cheated on by her boyfriend with a mutal friend of ours, I knew, and didn't say anything. Things have never been the same between me and friend A. 5 years have passed and she still brings it up. I have appoligized profusily, but she tells me it still hurts. Obviously she isn't over the fact that she was betrayed by her boyfriend, but she feels that I betrayed her as well.
Now there is the situation, that they could look on you as a sort of "home wrecker" I told my cousin that her fiancé was cheating on her, and even proved it, but I was the one in the fault, trying to break up her happy relationship. She didn't talk to me for months.
SO I think you really need to look in your heart, and do what YOU think is right. Maybe you could accidentally leave some sort of proof out in the open for Aunt M to find on her own...

I absolutely agree with "Aurora." If you tell your aunt you may very well be looked at as a home wrecker, someone who didn't mind her own business. She also may not believe you. If you don't tell her and the truth comes out, she finds out you knew, she will be very angry with you and feel like a fool.

Do you have proof of any of this or is it all hearsay?

I'm an upfront person - I'd look into my heart and if I feel I HAD to tell her I'd start with, "I don't know what to do about this, I don't want to hurt you but this is what I heard ..."

I agree with Judy and Aurora.

I suspect that Aunt M senses something is not quite right with Uncle M. Most likely if he is this heavily into sexual communication with their high school buddies to the point of them blocking him, he has more problems going on than Facebook.

Including not taking his anti-depressants. He was probably on them for very good reasons, and now he is acting out.

I too was faced with a woman I was counselling once, who disclosed that she was having an affair- with my best friends husband! You have no idea how I wrestled with that. Truly and completely torn in two. I kept my mouth shut, and she eventually found out and divorced him. I could probably have saved her a lot of grief.

I think a gentle approach without directly exposing his activities. Something along the lines of that you are worried about his mental health, and see if you can't gain some insight as to how she's been dealing with him. If she seems to know more, even about the Facebook thing, then spill the beans.

Otherwise, at least you have planted a seed, and when spaghetti hits the fan, she will know that you tried to warn her.

Very difficult position to be in, but I would be considering the Facebook thing as a symptom of much more that is going on.

I don't really have much evidence this time around but several years ago I was on their computer and this girl kept IMing trying to talk to my uncle. I was only 14 and, although it seemed suspicious and the thought crossed my mind, I didn't really believe it could be so. He always puts her down-she reacts like its funny and like he's just joking but I think sometimes it really hurts. I know it would hurt me.

It sounds like they are both in denial. Aunt M about his behaviour, drinking, and not taking his anti depressants, and Uncle M about the extent of his inappropriate communication with people on Facebook. I suspect he has more going on the computer than Facebook. He will eventually find somewhere where he isn't blocked.

At some point Aunt M will have to face reality, and most likely somebody telling her point blank that Uncle M has some serious problems going on here.

I do think that this is beyond your well intentioned thoughts of helping her by opening up all these cans of worms.

Most likely, because you are so close to each other, she will at some point confide in you. That is where you can do the most good.

Until then, keep on doing what you're doing. Stay close to her, and let her know like you always have, that you'll always be there for her no matter what.

p.s. Aunt M may be keeing this to herself as to not rain on your parade with the wedding coming up too.