To read and see photos of what your "friends" ate for breakfast, lunch, dinner and snacks in between. To know when they went to the gym and for how long. To know when they wake up and go to sleep. To view hundreds of photos of their kids, cars, scenery, and douche-y duck faced photos of them. To inform you that "I'm so mad at someone right now" and then saying "I don't wanna talk about it" when you ask them what the hell their problem is. And last but not least, to post random intellectual, deep and philosophical quotes that make them seem like they know what their mission and purpose in life is, and they understand the whole universe now.
To be with your friends on an online site. I would rather be with friends in person; not on a computer. But, I'm much older than most, so guess it's just another place online to chat, email, and have fun.