I wouldn't do anything. There's no knowing if it's a sick joke or anything about the person or situation. I don't think anyone is going to post a serious suicide mention, anonymously; if they are hoping for help, they're going to let someone know in some way where they actually are in real life or the note will be posted where someone knows who they are, etc. If I don't know who they are and the situation, I am going to dismiss it as "not meant for me".