Awww come on, I know you can guess...the Facebook app of course. I love the interface of the app, I haven't been on FB on the computer since I got the iPad.
The only problem I have with it is you don't have the ability to share a post or photo. Or if it does exist I can't find it.
I can,however, save a photo and then repost it, tho this makes me feel a little like I am plagiarizing.
But, until I can find a way to "share" instead of "take", hey, a girl's gotta do what a girl's gotta do ;0)