"many of your friends are currently online now" - You see, when you look at your friends' pictures, if they are online it will actually SAY next to their picture "currently online". Or have a dot that represents the same thing, whatever. The site is letting you know that many of your friends are "currently online" at this very moment. Still redundant, still incorrect, IMHO.
Your second example has no explanation for its redundancy. You would say "I am currently online" or "I am online now" unless what you are is referred to as "currently online" and not just simply described as such.
In the third example, I could see if an online service had a repository of many more books than they have room to keep "currently online" at any given moment. If you were allowed to choose which books you wanted to be available to you, after you chose they might tell you that "these 300 books are now currently online" and actually be (almost) grammatically correct.
Sorry if this didn't make sense; I'm getting a bit sleepy while writing it. Hope it helps.