Basically, you go there and hang out and get nothing worthwhile done, being seen by other people who are there to hang out and get nothing done and be seen. You poke at people as a "hey, what's up" - just enough conversation to be polite, but without all the deep connotations and actual thought:
Invaluable if you're into pokewars like I've somehow become.
There are people who are there who probably shouldn't be there (your parents, aunts, uncles, teachers) who you do everything in your power to avoid publicly interacting with without outright offending. Just like when you were at the mall and you'd pretend to not see them when they wave at you in Macy's, you pretend you never got their wall post (usually by deleting it without response and blaming it on "the damn redesign") or ignoring their pokes or removing your name in tags on photos they post. And then, every so often, you hop into the arcade and play some games.
There are stupid people asking other stupid people to sign up for stupid raffles where they can win stupid prizes:
There's even the same stores as the mall: Cinnabon, Panda Express, The Gap, Pac-Sun, Hot Topic, Bath & Body Works, Victoria's Secret (which you browse by but never actually go into or join)... There's even those stupid annoying retarded no-one-ever-visits-it-but-somehow-they-still-afford-being-there cellphone kiosks dotted around the Facebook Mall:
It's funny how, no matter how much technology changes and advances, we as a species just don't. Not in any fundamental way, anyway.